A DMX Receiver is any electronic item which can recieve, and decode a
DMX-512 signal. This is most often used to control lighting and other
dynamic objects in a play, concert, or other kind of show. The DMX
receiver can then take this signal, and select the channels that it is
meant to be listening to, and then do whatever it is meant to do with that
channel level.
The most common type of DMX receiver is a dimmer, although moving mirror and moving-head lights are also commonly found in concerts. These can take anywhere from 5 up to around 14 (or maybe even more) DMX channels per light. This is to control different things like the rotation, colour, gobo, etc.
If you're after any other theatre tech information, then do take a look at 'Everything you ever wanted to know about theatre tech, but were afraid to ask.'