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The AP polls are very telling, and very important for college football. They are updated every week during the regular season. They are used to help determine who goes to the playoffs, and which teams play in which bowl games.
But they don't tell you where a team is headed. They just rank the best 25 teams in the country as of right now, based off of what they have done so far - wins, strength of the wins, how good their opponents were in the those games (how impressive the win, or loss, was), etc.
So if a #17 team beats a #2 team, the #17 team goes up a lot after that win, because they beat a very highly ranked opponent. And the #2 team would fall a good bit, because they allowed a #17 team to beat them.
So the short answer is AP ranks are a snapshot of where teams rank today, but not necessarily where they will end up at the end of the season. Its updated every week. But if you make it into the top 4, you are a playoff team. So that's where it becomes most important.