So according to The Dragon Prince wiki, Callum's name is Gaelic for "Dove King" but that it can also be derived from the Latin word caelum, which means "sky." Since Ancient Draconic in the show is identical to Latin, that means Callum's name probably means sky in-universe which would be an amazing coincidence there.
She'd looked everywhere, but Callum and Ezran were nowhere to be found. Ezran rarely had time to himself since the three of them returned to Katolis. Rayla and Callum dedicated themselves to spending as much time with Ezran as they could to make up for it. That way they could allow the young king a chance to still be a ten year old in-between all his royal duties. But once court was adjourned for the day, she couldn't find him anywhere. Callum had vanished as well.
Having scoured the the throne room, the bedrooms, the council chambers, the kitchen, even the dungeons, Rayla was finishing up her search of the ramparts with nary a clue as to where they'd gone. Rayla took one last look around and noticed the shorter of Katolis' famous towers. It hit her at once. She'd checked inside the towers, but not their summits.
At the top of the lower spire, shielded from the world, Callum and Ezran huddled around a large, dusty tome. Bait laid off to their side. The trap door leading below slammed open, causing the boys their and glow toad to jump.
"There ya are! I've been lookin' everywhere," Rayla said.
"Oh, sorry," Callum gave an apologetic smile, "I was helping some of the guards clean out Viren's old study and I found this." He grabbed the book and held it up for Rayla to inspect. "I ran into Ez while carrying it and I was so excited I forgot to tell you. It's a translation guide for Ancient Draconic."
"Kind of like a 'Draconic for Dummies.' Callum's going to use it to make new spells!'" Ezran added.
"We went up here in case anything goes wrong, like if something explodes. Not that it will."
Rayla gaped at them. "Make... new... Ya can't just go and make new spells! Tha's not how it works!"
"Of course it is," Callum countered. "How do you think the first mages invented the spells that exist now?"
"They were taught them by the dragons."
"Then who taught the dragons?"
Rayla opened her mouth to retort, but realized she had no answer.
Callum grinned smugly and continued on. "A spell is basically just a word or phrase in Draconic, and a corresponding rune, right? So all I have to do is use this book to make a spell name, and then I'll just keep drawing different runes until I found one that reacts." He and Ezran looked up expectantly at Rayla.
"I..." Rayla paused, trying to compose her thoughts, "...I can't decide if tha's brilliant or stupid."
"It's brillpid!" Ezran said.
Callum frowned. "Thanks for the vote of confidence." He set the book down to resume his work.
Rayla shrugged and sat down next to them.
"I've got an idea," Ezran said. "Let's make a spell that makes bugs bigger!"
The others, even Bait, stared blankly at him.
"Why would anyone want to do that?" Callum asked.
"Look at Bait. He's adorable and perfect in every way." Ezran gestured to Bait, who batted his eyelashes in the closest attempt a glow toad could make to being cute. "But if Bait was the size of an ant, he'd be really creepy. People are grossed out by bugs because they're small and hard to understand. But if bugs were the size of normal animals then people wouldn't be afraid of them anymore."
"I'm gonna have to agree to disagree on that one," Callum said. "Besides, I don't think I can use Sky magic to grow anything."
Callum's mention of the Sky primal jogged something in Rayla's mind. Ever since Callum expressed his desire to connect to the Sky arcanum, something had bugged her. Something about Callum being interested in Sky magic in particular, but she couldn't put her finger on it. As his skills as a mage grew, that sense deepened. Callum bringing it up now in conjunction with the book in front her finally supplied the missing piece of the puzzle.
"Can I see the book for a minute?" Rayla asked.
"Sure."
Callum slid the book over to Rayla, who began to furiously thumb through the pages.
"Aha! I knew it!" Rayla stabbed her index finger against the page she was currently on.
Callum and Ezran were completely lost. "Knew what?" Ezran asked.
"The Draconic word for 'sky' is Caelum. Switch out the 'e' for an extra 'l' and ya get-"
"My name," Callum interrupted. He'd realized what Rayla was getting at.
"Yer name means sky and yer a Sky mage!"
Callum leaned over to see the page for himself. "Whoa. That's really weird; almost creepy."
"Dad told me my name means 'helper,' and I do like helping people. Maybe everyone's name is connected to them like that?" Ezran suggested.
Callum stared down at his hands, as if his entire worldview had been shaken. "I guess, but that's just really uncanny."
Ezran decided to live up to his name and help jar Callum out of whatever existential crisis he was going through by changing the subject. "Does your name mean anything, Rayla?"
Rayla smiled and nodded. "Yep. It means 'ray of moonlight' in an old Moonshadow dialect."
Her answer succeeded in diverting Callum's attention. Something else now occupied his focus and he couldn't keep it to himself. Callum flashed Rayla a teasing, lovestruck smile. "So does that mean you're-"
Rayla instantly realized where Callum was going with this, and did not approve. "Don't. You. Dare."
"-my Rayla of moonlight?"
Ezran snickered and Bait turned a mixture of pink and orange that meant about the same thing.
"Okay. Tha' was yer one freebie. Call me tha' again at yer own risk."
I thought about it going deeper and have them ruminate on whether Callum being a Sky mage is just a funny coincidence or destiny and then a larger debate about destiny versus free will but it was too serious for a silly one shot about name meanings. Also it would have been a bit of a tonal whiplash to go from that to "my Rayla of moonlight" for the ending.