DDoS means "Distributed Denial of Service"
A denial of service attack involves flooding a target with so much network traffic that it can't function. The process is akin to constantly calling someone's phone to prevent anyone else from calling it. It's hard to do so with just one attacking computer, though, so attackers generally use exploits to gain control of a large number of systems. They then have all of them flood the target at the same time.
There's no real way to defend against a DDoS- you can't filter out the malicious connections, as doing so means they're still wasting your bandwidth.