Distributed denial-of-service attacks are a family of attacks that cause websites and other internet-connected systems to crash by overloading them with traffic. The “distributed” part means that other insecure computers on the internet—sometimes in the millions—are recruited to a botnet to unwittingly participate in the attack. The tactics are decades old; DDoS attacks are perpetrated by lone hackers trying to be annoying, criminals trying to extort money, and governments testing their tactics. There are defenses, and there are companies that offer DDoS mitigation services for hire.
Basically, it’s a size vs. size game. If the attackers can cobble together a fire hose of data bigger than the defender’s capability to cope with, they win. If the defenders can increase their capability in the face of attack, they win.