A distributed denial-of-service (DDoS) attack is a method of rendering a website nonfunctional, making it unavailable to legitimate users.
A DDoS attack takes a website down by flooding the targeted server with traffic, overloading it to the point of inoperability. It’s like thousands of people trying to cram themselves through a doorway all at the same time. It makes it so that no one can get through the doorway, including people who have a legitimate reason to pass through to the other side.
Attacks like this are usually coordinated across a large number of client computers and other network-connected devices which may have been set up for this express purpose, or more likely have been infected with a virus that lets someone remotely control the device and enlist it in the attack.
Because the attack is coming from so many different sources, it can be extremely difficult to block. Imagine, again, the hoard of people cramming the doorway. Simply blocking one illegitimate person (or malicious traffic source) from getting through won’t help since there are thousands of others to take their place.
The best way to defend yourself from a DDoS attack is to prevent it. The most important thing you can do to prevent this type of attack is have a system in place that can differentiate between malicious and legitimate traffic. There are a number of security solutions that can help, like web application firewalls and DNS services.