Play two notes that are very close in frequency — say 440 Hz and 442 Hz. You'll hear a wobble: the volume rises and falls 2 times per second. That's a 'beat'.
Beat frequency
|f₁ − f₂|
The wobble rate equals the difference in frequencies.
Piano tuners use beats. They listen for the wobble between two notes; when it stops, the notes are perfectly in tune.