An automatic sell order that limits losses – your insurance policy.
Example
You buy SOL at €150 and set your stop-loss at €142 — about 5% below, just under a support level. If price reaches it, the exchange sells automatically and your loss is capped at that 5%, whether you are at the screen or not. Without a stop, a single bad trade can wreck an account. Rule of thumb: place the stop where your trade idea is invalidated — not where the loss merely starts to hurt.