Raid protection watches for two kinds of attack and tells you the moment it sees one. Nuke watch looks for an account that deletes channels, roles or members too fast. The join raid watch looks for a flood of new accounts joining at once. Only the server owner can turn it on, with Turn on in Configure or Turn on raid protection on the Safety page. (Configure → Discord safety.)
An alert goes to a channel you pick on the Safety page and to the server owner's DMs, and says what happened. Pick how strongly the join raid watch responds, starting from Notify only. Every incident is listed on the Safety page under Recent incidents, with Recover to walk you through getting the server back: first its structure, from the snapshot before the incident, then members' roles. Nothing runs between the steps until you press the next one.
The Safety page also shows Discord's own protections (like the verification level, which you set in Configure → Discord safety), Who holds power in your server, and Is Packet ready?, which checks Packet has what it needs to help. Freeze the server stops members posting, reacting and starting threads everywhere at once in an emergency. Staff and Packet still can, and tickets and voice rooms Packet made keep working. When it ends, every channel goes back exactly as it was.