What does a "rough in" crew do?

Easiest way to explain this is to give you some examples of some rough in work.

Electrician:  After your house has been framed an electrician will come through and place electrical boxes for outlets, switches, cable, phone, lights, ceiling fans and etc.  They will then run wire connecting all this stuff together and will eventually connect it to your electrical panel.  This is the rough in.  They will come back after the house has progressed to hang lights, put in outlets and switches, attach wall plates, and etc.

Plumber:  After your house has been framed a plumber will come in and run all of your water lines and drain pipes.  This the rough in.  Once the house has progressed they will come back and install the toilets, faucets, laundry shutoffs, and etc. 

A rough in crew is getting the less showy part of the job done.  All work done in a rough in stage will generally be hidden by the time the project is complete.