If you're confident your favorite team will win the championship even though the star player is injured, you may want to reconsider: don't let your team spirit cause you to overestimate, or overvalue, the team's abilities.
When you estimate something, you make a calculation or a judgment of its value. So, when you overestimate, you assign too much value to the thing you are judging. If you overestimate how much time you have left to get to school, you could be late. Likewise, if you overestimate how prepared you are for a test, you might end up with a poor grade. This verb is the opposite of underestimate, or “to assign too little value.”