Fishing up tight to a snag where you can't let the fish take any line off the reel, nor allow line stretch to let them get into the snag. It can be fishing to a treeline with roots growing into the lake, an obvious dead branch in the water, a serious weedbed or lilypads.
I fish to snags slightly differently to @kevtaylor, by pointing my rod directly at the snag, line tight, and the baitrunner switched OFF. It only works with mono at short range, as mono has about 15-25% stretch, so as Kev says, look at braided mainline if at any range.
The theory of the tight line is that on the take the fish is pulled in an arc away from the snag, then you can get back to playing it normally.
I'd also say, don't snag fish at night, or if you are sleeping. You need to be instant, right next to the rods at all times. Even straying away for a pee can be enough time to lose a fish or rod!