A Distributed Denial of Service (or DDoS) attack is a malicious attempt to deny legitimate traffic to a targeted server by flooding it with spurious requests. The attacker often accomplishes this by infecting a fleet of servers and internet-connected devices with malware in order to turn them into a botnet. This botnet is then instructed by the attacker to repeatedly send requests to the targeted server, overwhelming its available resources. This results in a denial of service to normal traffic.
For more information on DDoS attacks and how to mitigate and recover from them, please see the following documentation and tutorials: