What does fouling out mean when talking about spark plugs?

 

It's similar to short in a wire, when the plug gets something on the insulator like fuel, it will allow the current to flow to the ground instead of sparking across the gap like it should.

 

If there were such a thing, I would have won it in December!
These plugs definetely get the Krusty Brand seal of approval...
They looked worse than the 10 year old plug in my 2-cycle lawnmower... Number 4 was totally shorted.

They came from my old Ford 4 banger after the alternator quit and the choke stayed closed, causing an unusually rich condition for 60 miles...