Octaves beating is not specific to harmonica and is indeed a tuning thing. However, on the harmonica is it just as useful, when tuning harmonicas to check the consistency of all the notes.
When octaves are close to being in tune with each other but not perfect you will here a ‘beating’, I’d say it best described as a tremolo type effect, you need to listen closely to hear it, and as indeed any piano tuner will tell you, it is an art to be able to use your ears and tune like this..
The ‘beat’ can either be slow of fast depending on how close to the right tuning you are.
Hope that helps!