Slippage is the difference between the price you clicked and the price you received. In calm, liquid markets it is usually zero or a fraction of a pip. It grows when prices move fast or orders at your price run out.
Around major news, such as rate decisions or employment data, prices can jump several pips in a second and liquidity briefly thins. Market and stop orders fill at the next available price, which may be far from your level.
Market and stop orders can slip because they fill at the next available price. Limit orders can't fill at a worse price, but in a fast market they may not fill at all.
Latency is the time between your click and the fill. Even milliseconds matter in fast markets, because price can move in that gap.
A stable connection and a broker with servers close to major liquidity centres help keep latency low.
Slippage isn't always against you. If price moves in your favour between click and fill, you may get a better price than you expected.
A fair broker passes positive slippage on to you, not only the negative kind. It is worth checking its execution policy.
Slippage is normal. Plan for it.
