318 timing

I read that the timing for my truck is 6 degree's. It didn't say btc or atc. I found it off autozone website. Well I check the timing because I am getting a backfire/pop at idle speed in park and when driving during deceleration. I set the timing light degree dial at 6 and the mark lined up with 8 degree's btc. When I set the dial at 0 it was about 10 degrees BTC. I set the dial back at 6 and turned the distributor until the mark on the dampner lined up with 0. the pop is still there.

Can someone explain to me in laymen terms the results of advanced timing and retarded timing. I am assuming advanced is BTC and retard is ATC.