DDoS is short for Distributed Denial Of Service. It essentially eats up your bandwidth, making your system inaccessible to yourself and/or your customers. This can be used for many things such as distracting your security staff, protesting against your company (known as "hacktivism"), slowing down or even stopping your business.
What does this mean?
I like comparing computer systems to real life situations as this often makes for a great analogy. In this instance, imagine you are a shop owner and there are hundreds of people (or more depending on the size of your store!) stopping you and your customers getting into the shop. Meanwhile, these people have a friend that has popped round to the back door, gets himself in and has access to all of your goods, your customer records and anything else you have stored away in there.
This is essentially how a DDoS works. You may think "How can there be that many people attacking my servers at once?". This is a very valid point! There is very little chance of there genuinely being that many people willing to try and take down your servers. However, hackers can do it all by themselves. This is through the use of them spreading a virus around a network, transferring from person to person through email, software downloads and many more methods. The hacker can then use all of these PCs to launch an attack at your servers all at once. It's like all those people stopping you getting into your store are a horde of brainwashed people that are told to stop you and can't do anything about it.
Of course this is just one reason someone may attack your system and only one method too! Hackers may try to extort you or just stop your business running. Some even do it just for fun, with the only reason being to annoy you and your customers. Whatever the reason, these attacks can be very costly to your business, both in terms of time and money.
How can I stop it happening?
There is no way of becoming truly safe from a DDoS attack. The best thing you can do is ensure you have the necessary preventative measures. If your network is scalable this should stop low-end DDoS attacks. Only 24% of attacks are more than 1 Gbps and only 9% of those are over 10 Gbps. If your server can handle a larger amount of requests then you are a lot safer on most cases.
Your ISP may also have preventative measures such as early warning systems in place and may be able to increase your bandwidth in these cases. It is good to get them on-side and build your relationship with the provider to ensure you know the best contacts and what to do in these situations.
You can also get mitigation services which are known as "scrubbers". These services analyse all incoming traffic and can detect anomalies. This can then pass any odd traffic to a mitigation server where it can be dealt with safely, alert your security team and take any preventative measures automatically. Once the traffic has been checked it continues to your server as usual.
It would be remiss of me to not mention that there are other types of DoS attacks as well as DDoS as your systems can be overloaded in many ways, such as targeting specific parts of your system that take up a lot of resources and more complex techniques like having your IP address spoofed and making requests to a vulnerable DNS server.