A point spread is the predicted scoring difference between a game's two opponents. If you choose the team that's favored, they must win by more than the spread number in order for you to get credit for a correct pick. If you choose the underdog team, they have to lose by less than the spread number in order for you to get credit for the correct pick.
So, if you pick Penn State and they're favored to beat Notre Dame by five points (Penn State -5.0), they must win the game by six or more points in order for you to get credit for the win. Otherwise, if Notre Dame loses by less than five, you wouldn't get credit for the win. If Penn State wins by exactly five points, the game is a push, and no users get credit for a victory.
If a point spread is listed as "off," this indicates that there is no official point spread for the game. This is most common when a Div. I school plays a Div. II school. When the line is "off," you're simply picking which team will win the game.
Yahoo! Sports uses the point spreads provided by the Associated Press. These spreads will be updated each day, with Thursday's lines serving as the game's official spread.