An acciaccatura is a note which is played as fast as possible. It means a “crushed note” (in Italian "acciaccare" means "to crush"). It is normally written in small print but with a slash through it to show that it is not an appoggiatura. Most performers play acciaccaturas exactly on the beat, but sometimes it is better to play it just before the beat so that the main note is exactly on time on the beat.