Remote Exec lets you ship python code to a remote machine and run it there, all without installing anything other than the standard Python interpreter on the server.
It connects to the remote host using SSH, sends the python files you specify, compiles them on the server, and passes control to the specified main function.
Additionally, the client arranges for stdin/stdout on the server side to be connected to a network socket on the client side, so that you can communicate with the uploaded server binary as if you'd connected to it normally.
What this lets you do is ensure that the server end of your software is never out of date, since it'll always be in sync with the client code you're running.
The original idea came from Avery Pennarun's awesome sshuttle (http://github.com/apenwarr/sshuttle), which uses this great hack to function with any server that has Python installed (i.e. all of them).