Music is divided up into bars (sometimes called measures)
as shown below.
Barlines are vertical lines placed along the staff
Each bar will have the same number of counts, or beats, so that we can play the music in the rhythm or timing that the composer intended us to.
Notice the two numbers after the treble clef ; 4
These numbers are called the Time Signature.
The number on top, tells us that there are four beats or counts in each bar.
The number below, tells us what kind of note is worth one count.
In this case, it is a quarter note. So the time signature tells us that there are four, quarter note beats to each bar.