Music is counted. A bar holds exactly 4 beats - no more and no less.
The shape of a note tells you how much it counts - a whole note 4, a half note 2, a quarter note 1.
At the top you see the notes in the supply, with its beat count under each one. All of them go into the gray boxes, each one exactly once.
How many notes fit into a box is open. If 2 beats are missing, one half note works, or two quarter notes. Only one way of sharing uses up every note.
Example
For parents
What does this exercise practice?
Filling bars makes the decomposition of 4 visible instead of just claiming it. The child sees that 4 = 2 + 2 = 2 + 1 + 1 = 1 + 1 + 1 + 1, because every note is printed exactly as wide as it counts. And because the supply is shared by all the bars, the child has to think about them together rather than checking them off one by one. If you like, clap the finished line out loud.
Common pitfalls
A box is not always a single note. Putting exactly one note into every gray box usually does not add up to 4 - often two or three notes belong there.
How to practice at home
Clap four times, always at the same speed - that is one bar. Now clap once and hold it while counting to four (whole note), then twice for two beats each (half notes). What lasts the same time?
Related: Splitting Numbers, Number Wall