what it does is floods the server with requests so when someone else tries to go on the server is already overloaded with requests and won't load
servers work like this
you send a request, or a "hello"
the server responds, "Hi"
you send a request for the webpage, or a "wassup mang"
the server sends you the data, or a "not much mang, u?"
then your page loads
but a DDoS is more like this
You send a hello
the server sends a hi
you send a hello
you send a hello
you send a hello
you send a hello
you send a hello
you send a hello
ect. ect. ect.
and the server tries to respond back, further overloading it