DDOS Stands for Distributed Denial Of Service (attack). A DDOS Attack sends more data to a target, like an IP Address, or a website, than it is actually capable of handling. Let's say a website has a global network capability of 10GBPS, in and out (which is very fast). You can send around 15-20GBPS, and if the target does not have a DDOS Mitigator of some sort, the target will be offline for however long you send the data for.
But, you may be asking, "What is a mitigator, and how do I get one?"
A DDOS Attack Mitigator is a system that attempts to completely suppress a DDOS attack, by blocking suspicious traffic with large amounts of data with the same pattern every time, and same info, and the same port. Let's say you use a hosting company for your website like OVH, and your OVH Web Server has a speed of 100MBPS in and out. Someone can easily send 1GBPS to take down your network, but after the attacker sends the 1GBPS Attack, OVH's mitigator will kick in and ensure that the network is about 100% filtered, and immediately block any traffic which is large or traffic that would affect the network without the mitigator.
As you can tell, mitigators are VERY useful in lots of situations, including HTTP Floods/layer 7 attacks, IP-based DDOS, and/or DOS attacks/layer 4 attacks, and many more methods to slow down a network.
Fortunately, company's like OVH do offer mitigators for free, and when you purchase web hosting for your website, or a virtual private server or dedicated server, you should always ensure there is a mitigator.