Slippage refers to the difference between the expected price of a trade and the price at which the trade is executed. Generally speaking, this is due to the price change from the time the order was placed to the time it was filled. For example, let’s say you place a buy order for EHT on a DEX, and, of course, you want to close at the price you set.
But when the trade is executed, the final price is higher or lower than the price you chose when you placed the order. That is a slippage. If the buy price is higher than the price when you placed the order, it is a negative slippage in which the transaction fee will be more than expected. A positive slippage is when the buy price is lower than the price when you placed the order. And vice versa.