It is simply slang for the second weld bead deposited in a joint between two pieces of pipe.

Al

There are many different reasons as to WHY the 'hot pass' is run.

It performs a bit of a heat treat/tempering of the HAZ.
It can slightly 'push' the root pass farther through to give a good contour to the ID of the root pass.
It add thickness to the weld that needs to be accomplished before welding is allowed to stop at the end of the shift. 
The added thickness aids in preventing cracking as the weld cools. 

He Is In Control, Have a Great Day,  Brent

Two more (and very important) reasons for a "hot-pass":
1--When welding open-butt root passes with GTAW, it is often required to weld the fill and cap passes with another process such as SMAW. In this case, you might burn a big old hole right through your shiny new root pass unless you run a GTAW "hot-pass" to beef it up a little bit first. I'm speaking from personal experience here...
2--When welding open-butt root passes with SMAW (usually 6010), a "hot-pass" with the 6010 over your root pass helps to burn out any imperfections (like "wagon tracks") before going to your fill and cap passes, (again speaking from personal experience...)