What is a DDoS attack?
A DoS ("Denial of Service") attack, is an attack vector whereby a single or few host IPs send specially crafted malicious traffic towards a server or host on the internet i.e. that target, with the intent of making that target unavailable for others to use.
For example: if a website has a DoS attack launched against it and the DoS attack is successful, that website will not be available for users to connect to.
A DDoS ("Distributed Denial of Service") attack is the same concept with the main difference being that the attack originates with a more distributed attackers base, i.e. from many source IPs and generally multiple geo locations. It can be hundreds or even thousands of source IPs where the attack originates from. This gives the attacker the advantage of making it more difficult for the targeted victim to mitigate the attack.