DDOS stands for Distributed Denial of Service. basically in the minecraft sense, this means that hundreds of fake accounts attempt to connect to the server, causing the server to fill to 100/100 slots, not allowing other players to join. these attacks are sometimes accompanied by spamming in chat.
as far as prevention, we started using a modified iptables firewall after the first DDOS attack to drop connections from the same ip if they go over a certain amount of connection attempts in a given time period. as far as i know, it's been working rather well.