Pots Question

[KevScot]

Hi I've just put this together on a bread board using the components listed and it works. 
Much to my surprise as it's my 1st attempt.
I'd like to know what determines the resistance of the pots in this circuit.
I'm sure this must be covered somewhere in the forums and I'd be grateful if someone
could point me in the right direction.
Thanks for a great site.