Team A is a real powerhouse. Team B is promising but inconsistent. When teams A and B meet, A has the clear advantage. So the sports book might set a point spread of 10 points on the game. That would mean the favored team -- team A, in this example -- would need to win by at least 11 points in order to "cover" the spread.
The sports book will manipulate a point spread to ensure that there's a fairly even distribution of bets on both teams in any contest... which ensures that the house makes money no matter the outcome.
For a more detailed explanation of point spreads and how they are set, check out Allen Moody's Sports Gambling 101.