Sliding scale pricing is an option you can choose when pricing a class. You'll see it listed next to the default 'Fixed' price option.
It can be viewed as a donation opportunity for customers of higher means to give back, or a more inclusive offering, allowing those that can't normally afford to pay to join.
To use it, select it and set the lower and upper limit. Save changes.
Now, on a customer facing class sign up page, customers will now see a sliding scale bar. The marker starts at the middle of the range, allowing customers to drag it to where they feel comfortable paying.
And, you'll also see this option when ringing customers up for that class using POS, allowing you to ask the customer what they'd like to pay.