DDOS is an acronym for Distributed Denial Of Service attack.
The concept is fairly simple: a distributed network of computers on the internet, typically a “botnet” of infected computers, are all instructed to access a specific website or service simultaneously. When faced with the overwhelming load attempting to service thousands or tens-of-thousands of simultaneous requests, the targeted site is unable to service any, and is effectively forced out of service.
The term “distributed” is used because the attacking computers are distributed all over the internet.
“Denial of service” is the goal. The site is unable to service legitimate requests and users when overwhelmed by the attack.