Il a fait les cent pas pendant une heure.
ee lah fay lay sah pah pah-dah EW NUHR.
He paced up and down for an hour.
Literally: he did the 100 steps during an hour. Okay, so that’s really, really bad English. But it reflects two facts about the French language: that French likes to attach a specific number to an unknown, and that French speaks of an ongoing action in terms of the time elapsing.
More simply: Why cent pas? Because it sounds like a lot. There’s no good reason to choose that number, and most people who pace up and down aren’t counting their steps. It’s just the number that is used.
And pendant une heure: The word pendant means during, according to the dictionary. The idea of today’s phrase is to focus on the long time that our character had to wait, and what he was doing during that time. Using pendant, during, really draws our attention to the dreariness of the wait, how long and boring it was, and perhaps even how nervous our character seemed while waiting. So when you want to emphasize what someone was doing for a certain period of time, use pendant.
By the way, un pas is where English gets one pace from. It’s from the Latin passus, a step.