The default sort is "active" which means that posts that are still getting a lot of comments stay at the top. I assume "hot" is more Reddit-like but I haven't really checked.
"Active" sort means that the Front Page works more like an old school internet forum. Threads that are still actively being commented in and discussed stay at the top and get bumped up.
"Hot" sort is more like a Reddit sort, it's a combination of the user voting score of a post divided by some sort of time function.