Slippage tolerance is the maximum price deviation you will accept between requesting a trade and having it filled. Set it to zero and orders are rejected rather than filled at a worse price; set it wide and they fill, sometimes far from where you clicked.
The right setting depends on what the order is for. An entry can usually be rejected and retried, so a tight tolerance costs little. A stop-loss cannot: refusing a bad fill in a fast market means staying in a position that is still moving against you. Most platforms let you set the two separately, and most traders never do.
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Guide
Order Types and Slippage Explained
Market, limit and stop orders trade price certainty against execution certainty — plus what slippage is, where it comes from, and what a stop-loss caps.