"This is just how wizards get around," said Ron Weasley, stepping out of Harry's fireplace. "Come on, let's go back to my house."
Harry tossed a handful of floo powder, followed Ron into the fire and out of the Weasley household fireplace. "How does it work?" he asked.
...
Ron's dad said, "FLOO stands for fire-looping. It's a transmutation of location into passage. Every fireplace connected to the network is also a place-fire, you see, on the surface of a flat earth. The earth's not flat. But it will unfold, someday during your lifetime. And we can bend time a little to use the flatness ahead of time, for a convenient teleportation system."
"So does time stop when you're in between fireplaces?" Harry asked. Not wanting to face his prophecied battle with Lord Voldemort, he spent the next couple years within fire.