Slippage refers to the difference between the expected price of a trade and the price at which the trade is executed. Slippage can occur at any time but is most prevalent during periods of higher volatility when market orders are used. It can also occur when a large order is executed but there isn't enough volume at the chosen price to maintain the current bid/ask spread.
For example, you want to buy at 1.005 but due to low liquidity, the price matches up to 1.013.
Slippage often occurs on brokers that have a market execution system, so if you sell but no one buys, the price will move towards a more unfavorable price.
Slippage is completely unavoidable if you trade using market orders. A trader can minimize slippage is to ensure that the broker has multiple liquidity providers.