That's a finance question, not a MATLAB question.
Here is simplest (but I think correct) explanation I can come up with:
Basis refers to the day count convention on how the interest of the bond itself accrues. This is important for the exact schedule of the interest payments. (There is a description of that in the documentation here.)
The concept of "discounting" is that a payment later is not as valuable as a payment today. DiscountBasis is again a specification of the day count convention, but it is for the timing of discounting of the payments, not the timing of the payments themselves.
I hope that helps.
