A bull put spread (sell a put, buy a lower put) profits if the stock stays above the short put. A bear call spread does the reverse. The credit is your maximum gain; the width minus the credit is your maximum loss.
Credit spreads are high-win-rate, low-payoff trades that harvest theta and benefit from falling implied-volatility. The occasional full loss is several times the typical gain, so sizing matters.
Example: stock at $100. Sell the $90 put for $1.50, buy the $85 put for $0.50. Credit $1.00 ($100). Max loss = $5 - $1 = $4 ($400) if the stock is below $85 at expiration.
Related: vertical-spread, debit-spread, iron-condor, theta, iv-rank