A DDoS attack is a coordinated attack against a website designed to make the website unusable. The name DDoS is abbreviation of Distributed Denial of Service attack which describes both what it is trying to do and how it goes about doing it. The attacker utilises many, in some cases a very large number of, different computers around the internet to attack the target system. This gives the distributed part of the name as the attack is distributed amongst lots of machines.
The Denial of Service part indicates what the attacker is trying to achieve. What they are trying to do is to stop your site from being online and working normally for your users, that is to say they are attempting to deny normal service to legitimate users.
These attacks usually takes the form of sending huge numbers of page requests or simply junk data which overwhelm the servers ability to service them. When this happens any legitimate page requests are lost in the attack and your site looks offline to anyone trying to visit the it.