When you read piano music, there are sometimes small numbers above or (usually) below individual notes. These are there so that the composer can tell you the best or most efficient way a passage should be played. How? Because those numbers correspond to which finger should play which note.
This comes in very handy for tricky passages where if they are not played with the correct fingers, are near impossible to complete. Almost all of the time, those numbers will make your life easier.
The numbering system is quite simple. It is the same for both your left hand and your right hand. The numbering system consists of the numbers one through five, with one being the thumb and five being the ‘pinky’ or little finger.