Re: what is spark plug gap?
Spark plug gap is the distance between the electrode and the ground prong. it's literally how far the spark has to jump. If it's too long, the spark is weak. Too short, the spark gets shrouded, and ends up weak again.
You change the plug gap with a gapper (imagine that). You can get em from any parts store. It's best to go with a wire-type gapper, cuz the ring-type ones can be pretty inaccurate. Just expand the gap by prying the prong away from the electrode GENTLY with the gapper. If you go to far and the gap is too big, tap the prong against a table, again GENTLY. You'll get better at it as you go.
Plug gap depends on how your car is built. If the engine is stock, then go with the recommended factory gap. If you've got something more than just bolt-ons, say, nitrous or a turbo, then you'll want to maybe call the turbo or nitrous company and find out from them what brand of plug and what gap is best.