Slippage happens when a trade executes at a different price than the one requested, usually because the market moved between the moment an order was placed and the moment it was filled. It's most common during high volatility or around major news releases.
Slippage can work against you (negative slippage) or occasionally in your favor (positive slippage). Market orders are more exposed to slippage than limit orders, which only fill at the specified price or better.
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Guide
Order Types and Slippage Explained
Market, limit and stop orders trade price certainty against execution certainty — plus what slippage is, where it comes from, and what a stop-loss caps.