We had an inspection done on our house last week. The inspector stated under a concern for electrical that our main panel box is not bonded properly to the neural system. What does this mean exactly and is this something that we can fix ourselves?

Recently I undertook to replace the standard recepticle on my outdoor patio with a GFCI. Upon removing the old one, I noticed that of the three attached wires, two were yellow (!) and one was white. I determined that one of the yellow wires was the hot. But I can't figure out which of the remaining two is ground and which is neutral (assuming that one is in fact neutral and the other is ground). There is continuity between those two wires, and a plug-in circuit tester reads correct both when the non-hot yellow is connected to the ground screw of the GFCI and white to neutral, and vice versa. How can I tell which is which? By the way, my knob and tune system is grounded throughout (circuit tester shows correct wiring at all outlets, and all except this one have the expected black, white and green). The grounding was accomplished by running ground wires to various cold water pipes.