An order that closes a trade automatically at a set loss.
A stop loss is an instruction to close a trade if price reaches a level you chose in advance. It limits how much a single trade can lose.
A bot that trades without stop losses, or that sets them very far away, can hold losing trades open for a long time. Its record may look smooth until one trade takes most of the account. Ask any seller whether every trade has a stop loss and how far away it is.
Educational content only, not financial advice. Trading forex involves substantial risk of loss.