The purpose of the lighting plot is to give you a design of the layout of the lighting rig for a production. It shows each light type, its position, its gel, patching point and often its patching cable length, angle and focus as well.
Here's a basic key:
http://www.gweep.net/~prefect/pubs/iqp/img53.png
You can do a basic lighting plot in Paint. However, for a good design, you'd probably need specialty software like Vectorworks. However, its quite expensive.