The credit spread involves two option legs, but results in an investor getting paid a premium to take on a limited amount of risk.
A 30 day call option at the 170 strike is priced at 3.25. And the 180 strike call option is priced at 1.00. The investor might sell the lower 170 strike and receive $325, and pay $100 to purchase the 180 strike. Since the investor is receiving more for the lower strike call than they are paying for the higher strike, the net cost of combining the two strike prices is a credit of $225. The combination in this order is called a vertical credit spread. Below 170 at expiration, the investor keeps the whole $225.
(IBKR, 2023, VERTICAL CREDIT SPREAD, https://ibkrcampus.com/trading-lessons/vertical-credit-spread/ )