Slippage means the dissimilarity between the expected price of a position as well as the price where the position is made. Slippage frequently happens while in periods of biggest movements, whenever market trades are utilized, also whenever large positions are made when there might not be sufficient interest at the preferred price level to keep up the estimated price of position. Also, sometimes only we might see Slippage, especially when any important financial or economic news released during news sharp rise/fall in the market price, and sometimes you open the position at the different price but due to sharp movement your order filled at the different price.
Same goes for your stop loss OR Take profit as well some time due to fast movement your stop loss will not close your position that you have set it will close your position the best available price of the brokers liquidity provider and also with your Take profit.
In Other Words: Slippage is the dissimilarity between the things you plan to pay for a stock or even ETF along with the actual price you get. It can also be the dissimilarity between the things you be prepared to sell a stock or even ETF but the things you get.