A teaser bet is a combination of bets on two different games. However, as opposed to a standard parlay, a teaser bet enables the bettor to adjust the point spreads (so instead of +5.5 for example, you could perhaps have +6.0, +6.5, or +7.0 points) for the chosen games.
The result is that you would theoretically have a higher chance of your bet winning, but you would have a lower return in the event of a win.