Slippage is the price difference between when you submit a transaction and when the transaction is confirmed on the blockchain. It can be caused by a few reasons – high trading volume, front-running, and low liquidity.
Low liquidity is when the liquidity pools for a given pair aren’t deep enough. This usually happens on less popular trading pairs. Depending on the size of your order (buy or sell) you may impact the price of the token itself resulting in the price being higher or lower than the one you confirmed. In order to avoid that, you should either trade with a smaller size or adjust your slippage tolerance so that the order doesn’t go through in case the slippage is too high.
Additionally, if your gas fees are too low, it might take a while before your order gets confirmed on the blockchain. During this time, while the transaction is waiting to be included, the price may move up or down due to the high trading volume.