DDoS stands for Distributed Denial of Service (DDoS) attack, and it’s when someone will try to make a website unavailable.
To perform a DDoS, you need many compromised machines, that in many cases are referred to as Bots.
Those machines at a given time will try to overflow the website with traffic using different techniques.
A successful DDoS is when the site is unavailable or slows down the loading time for the real users that try to reach them.
You can make an analogy where you are a waiter a can serve with ease five tables, but if twenty tables are full and all are demanding your attention at the same time you would yell: that’s enough I quit!
The exciting part of DDos today is that you can perform an attack with anything connected to the internet capable of receiving and sending information (packages) trough it.