How two tones make one wobble
Play two sine tones at 440 Hz and 444 Hz and you don't hear two pitches. You hear one tone that swells and fades. The two waves drift in and out of step. When their peaks line up, the sound is loud, and when a peak meets a trough, they cancel. The swell repeats at the difference between the two frequencies, 4 Hz here, or four beats a second.
The pitch you hear sits in the middle, at the mean of the two, 442 Hz. Two sines of equal strength add up to a tone at that mean frequency with a slowly changing volume.
Using beats to tune
Beats make a good tuning tool because they slow down as two pitches meet. Musicians listen for them to check a unison, a fifth or another simple interval, and piano and organ tuners count them, aiming for a set rate for each interval. Pull a string from 444 Hz toward 440 Hz and the wobble slows from 4 beats a second to 1 beat a second at 441 Hz. When the pitches match, the beating stops.
The same trick works against a reference tone. An orchestra tuned to A4 = 442 Hz beats at 2 Hz against a 440 Hz reference. A tone that's 4 Hz away from 440 Hz is 15.7 cents out, and a cent is a hundredth of a semitone.
Where beats stop sounding like beats
Beats only work when the tones are close. As the gap grows, the pulsing speeds up until it stops sounding like a wobble. Once the beat rate itself gets into the range you hear as pitch, the beating starts to sound like a note of its own, called a combination tone. At the other end, when the tones are nearly identical, the beating can become too slow to notice.
Beats versus binaural beats
A true beat needs the two tones to mix before they reach your ear, so the volume really does change. In a binaural beat, each ear gets its own tone, and your brain builds the sense of pulsing. A common example is 530 Hz in one ear and 520 Hz in the other, which gives a 10 Hz binaural beat.
That takes headphones. The Frequency Generator sends all its oscillators to both ears as one mixed signal, so what it plays are real beats.