Slippage refers to the time taken between the app fetching the current price and the user confirming their order at said price. This results in the actual price of the currency being purchased changing slightly.
Customers may experience this issue when trying to trade your full - or close to your full - balance in one trade. The actual price of the currency changed from what was originally quoted, so you no longer have the funds to cover the entirety of the trade, which caused it to fail. It is the high volatility of cryptocurrency markets that makes slippage more of an issue than conventional markets.