Slippage is the difference between the expected price of a trade and the executed price of that trade. It is more likely to happen when there is a higher level of volatility, such as breaking news that forces unexpected trends in the market.
This condition can make it extremely unlikely to be able to execute any trades at the expected price. Slippage can be a direct result of a speculator/trader using market-orders to enter or exit a position.
It is also likely to happen when large orders are triggered while there is a lack of volume at the selected price to maintain the current bid/ask spread. The spread refers to the difference between the ask and bid prices of an asset.
The difference between the mid-price and the execution price of a trade. You probably are familiar with the warning below before completing a big trade on PancakeSwap:
Increase slippage to 9/10% or 11%