Your alibi is evidence that proves your innocence. If you were making cookies with your mom when someone raided your sister's piggy bank, your mom is your alibi, since she knows you were with her when the crime was committed.
The noun alibi is the same word as its Latin root, alibi, which means "excuse." When you provide an alibi, you are giving proof — a certified excuse — that you could not have committed a crime. Alibi is easy to confuse with alias, which means "an assumed, or false, name."