It would be much easier to just break it down into chunks.
An example..
"What is a UDP flooder?"
Answer - Well we know the UDP protocol drops packets and is unreliable over TCP and is connectionless.
"What is a flooder?"
Answer - In computing the aim to "flood" is to send mass messages.
In conclusion.
UDP Flooder. = a mass flood of UDP packets.
Now onto your Question.
"What is a UDP shell?"
A shell can consist of one, two, three or several more types.
Two common ones in computer security, 1st one being SSH shells, which you would use to tunnel through in order to increase anonymity.
The second is a c99 shell, or c100 shell etc.
Which is used in web security to gain extra privelages.
Now I may be completely wrong as I've never dealt with web shells, I'm sorry if I made a mistake.
But in conclusion.
Web shell - (c99) + UDP = A shell being hosted on a hacked web server which uses UDP connectionless packets for something like Denial of Service (or whatever)
I may be completely wrong again.
But Google will really help if you break it down
Also, try to find some source codes of different "Shells" to learn the full whole as one