DDoS

A denial of service attack does not break in. It tries to use something up until nothing is left for anyone else.

Everything has a limit: how much traffic a connection carries, how many people a server can talk to at once, how much processing is available. Flood any one of those and the service stops working for real customers, without anybody ever getting inside. Attackers make this cheap by bouncing traffic off other people's machines so a small push turns into a large one. It is also occasionally a distraction, keeping everybody busy at the front while something quieter happens elsewhere.