Stop orders are submitted on the opposite side of the market than normal Limit orders. A buy stop order is submitted at a price higher than the current best bid but does not fill until that price trades. A sell stop order is submitted at a price lower than the current best offer price.
Note that if the market is moving rapidly, you may get filled at a worse price than the one you entered. If you want to prevent this by setting a limit price for the stop, you can do so with the Order Ticket.
Stops are useful as loss limits, allowing you to automatically close out a position if the market moves against you.