Whenever you see a piece of sheet music, there are all sorts of things on the sheet. Most are the notes and rests that are printed on the lines. However, there are other things on a sheet of music. In the far left, there may be one number over the other number. It looks similar to a fraction.
For instance, you may see a four then a line and then a number underneath it. These are called the time signatures. This means that there are four beats in a measure. This is probably the most common measure for most musical pieces. If there was a time signature of three over four, then that would mean that there are three beats per measure.