In piano lingo, fingering refers to using the best finger to play each note of a song, and correct fingering is always a very important part of piano playing. Even easy piano pieces have fingerings marked in the sheet music to help you plan which fingers to use to execute a particular musical passage most efficiently and comfortably.
The fingerings you see in music correspond to the left- and right-hand fingering in the following figure. Number your fingers 1 through 5, beginning with the thumb as number 1 and moving toward the little finger, or pinkie.
While you get used to thinking of your fingers in terms of numbers, you may find it helpful to write these numbers on your hands before you sit down to practice. Washable marker or pen is best unless you want to explain those numbered fingers to your date on Friday night, your boss on Monday morning, or your homeroom teacher.