A product has a set price when it is sold. When you buy that product with a credit card, you either pay for it in a single payment or in installments.
When a product is purchased in installments, merchants generally tend to add a certain amount to the price. (This is because banks charge a higher commission on installment transactions, or because the merchant only receives the product's price into their account in installment-sized portions.)
For this reason, merchants offer installments at the cash price in order to attract customers.
In short, it means selling a product to a customer in installments via credit card without adding any markup on top of the cash sale price.
For example, if a product worth 100 TL is sold for 100 TL whether paid in cash or in 5 installments, it has been sold for 100 TL as installments at the cash price.
Selling at installments at the cash price benefits the cardholder most of all. The cardholder gets the convenience of spreading out payments for their purchases over the coming months.