Simply put, slippage is the amount the price can change by while making a trade.
The price of tokens on uTrade is based on an algorithm which takes into account supply and demand of the two tokens in the uPair, based on available liquidity.
The algorithm can only display the price at the current conditions. The very act of making a trade has changed those conditions, so the price will have to update to reflect the new supply and demand. It is also possible other traders could interact with the liquidity pools while the user was setting up their trade. This change to the originally projected price is called slippage.
uTrade protects users from extreme slippage by enacting a maximum trade amount per transaction (expressed as a percentage). The larger the trade is relative to the size of the liquidity pool, the more slippage may occur.