For x/4 time, there are x amount of notes/rests in each measure.
In the example shown, the signature is 4/4, and there are 4 notes/rests in each measure. The first measure shown has two quarter notes and one half note, which takes the place of two half notes.
In 3/4 time, there are only three notes/rests per measure.
In 2/4 time, there are only two notes/rests per measure.
