What slippage is
Slippage is the difference between the price you expected when placing an order and the price at which it actually filled. If you click to buy at 1.1000 but the order executes at 1.1003, you have experienced three points of negative slippage.
Slippage is not necessarily a sign of a bad broker. In fast-moving markets, the price can change in the fraction of a second between your click and the fill. Sometimes slippage even works in your favour, giving a better price than requested — though people notice the unfavourable kind far more.
When it gets worse
Slippage grows when liquidity is thin and volatility is high — around major news, at market opens, and during sudden shocks. In these moments the price can jump past several levels at once, and there simply is no fill available at your requested price.
This is where the risk becomes real. A market order placed into a news spike can fill dramatically worse than the screen suggested a moment earlier. If you are trading around scheduled events, expect this and size accordingly.
Slippage and stop-losses
A crucial and often painful point: a standard stop-loss is not a guaranteed price, it is a trigger. Once triggered, it becomes a market order and fills at the next available price. In a fast gap, that fill can be well beyond your stop level, so your actual loss exceeds what you planned.
This is why traders can "lose more than their stop-loss" and feel cheated when in fact the market simply gapped. If certainty on the exit price is essential, some brokers offer guaranteed stops for an extra cost — but for most, the lesson is to avoid holding leveraged positions into known high-risk moments.