It is a unit of measurement when we are diving time in music. It can also be called count.
We have a natural sense to it. We tap our feet at a particular point in a song. Each of those points is the beat.
It is constant and doesn’t change. Its like heart beat. But heart beat can be fast or slow and that is tempo
A song can have 50 to 200 beats.
Without a discernible beat, you have nothing to dance or nod your head to.
A beat is a pulsation that divides time into equal lengths. A ticking clock is a good example. Every minute, the second hand ticks 60 times, and each one of those tick is a beat.