One cucumber costs $0.89, so with Erin buys "c" cucumbers, the price he will pay for the cucumbers is the unitary price (0.89) times the number of cucumbers ("c"), so the price is 0.89c.
One apple costs $0.99, so with Erin buys "a" apples, the price he will pay for the apples is the unitary price (0.99) times the number of apples ("a"), so the price is 0.99a.
Then, to find the final price Erin will pay, we just need to sum both prices: all the cucumbers and all the apples:
Final price = 0.89c + 0.99a
So the expression represents the final price (or cost) Erin will pay for all products.