....an amount equal to six months' advance interest on the amount.....

what does advance interest mean??

You need to ask the legal counsel that drafted the language or look elsewhere in the contract. I'm not sure how anyone can opine on a specific phrase not knowing the context from which it came.

this is the exact verbiage taken from the California Civil Code Section 2954.9.

A prepayment charge may be imposed on any amount prepaid in any 12-month period in excess of 20 percent of the original principal amount of the loan, but the charge cannot exceed an amount equal to six months' advance interest on the amount prepaid in excess of 20 percent of the original principal amount.

It means that if your original loan is for $100,000 and the customer prepays $25,000, then the penalty would be six months interest on $5,000 as that is the amount of the prepayment that exceeds 20%.