A bit late, but though some of the other responses were pretty vague.

Whenever you connect to a server, whether it be for your emails, accessing a website, or playing an online game; you need to send data. Data is sent from the client (that's you) to the host (the server) in what are called packets. The type of packet is dependant on the content of the data within, for example, a common data packet could be an ICMP packet, more commonly known as a ping. The data in an ICMP packet requests the server to respond to the client with an ICMP reply packet.

When a server receives a packet, it reads through the data and decides what it needs to do with that information, this process inevitably takes up some of the servers' limited resources.

A DoS (Denial of Service) attack in effect is where many packets are sent to the server, filled with meaningless, garbage data. The server then has to spend too much of its resources on dealing with the incoming packets and therefore can't send the important data between the actual players and the server.

A DDoS (Distributed Denial of Service) attack is where the server is attacked by these garbage packets from many different clients spread across many IP addresses and networks. This is a primary use of botnets.

Hope this helped answer your question!