What slippage is
Slippage is the difference between the price you expected and the price you actually got. Between your click and the broker executing, the market can move, so you fill a little better or worse. It works both ways, though people notice the bad fills more.
When it happens
Fast markets around news, the open, or thin liquidity. When a stop-loss triggers in a sharp move and fills at the next available price. Over weekend and news gaps, where price jumps with no trades in between. And with large orders that cannot all fill at one price.
How to limit it
Set a 'maximum deviation' on market orders so you only accept fills within a small range. Use limit orders when you want a specific price and are willing to risk not filling. Avoid entering and exiting in the most violent seconds of news. Trade liquid instruments in busy hours, where slippage is smallest.
How to plan around it
Assume some slippage on stops during fast moves and gaps, and size positions so a worse-than-expected fill is survivable. A guaranteed stop, where offered for a fee, removes gap slippage on that exit.
Broker's fault or yours?
Symmetric slippage in genuinely fast markets is normal execution, not manipulation. If slippage is always against you, or huge in calm conditions, that is a red flag — compare fills with another price source and consider whether the broker's execution is fair.