A credit spread sells an option and buys a further out-of-the-money one at the same expiration, collecting a net credit. Max profit is the credit; max loss is the strike width minus the credit; both are fixed at entry.
Bull put spreads (short put + long lower put) profit when the stock stays above the short strike; bear call spreads mirror that above the market. The long leg converts an undefined-risk short option into a defined-risk position, at the cost of part of the premium. Entry disciplines screen credit spreads on premium richness, short-strike delta band, liquidity, event dates, and a credit-to-width floor, with sizing computed from the worst-case loss.