1) Rhythm is a mixture of sounds and silences referred to as “notes” and “rests”.
2) Each note and rest has a value, denoting how long it should be held for.
3) How long a note or rest lasts for is described in "beats".
This is a Crotchet, also called a Quarter Note. It lasts for one beat. In practice, this means when you see a Crotchet, you clap once.
If you see four crotchets you play four times, each lasting for one beat.
Each Crotchet beat lasts for the same amount of time: one beat. This is why it sounds even and measured. If you were to play four times for different lengths, it would no longer be four crotchets and would be a different rhythm.
NOT FOUR CROTCHETS