A sloper is a hold without any kind of "positive" edge to it, which slopes down in the direction of pull. With good open hand strength you can get a surprising amount of grip out of sloping holds, though it will never feel quite as secure as a jug - you'll just get better at learning when you are, and aren't, about to slip off. Judging the best direction in which to pull on a sloper and adjusting your body position to allow yourself to pull in this direction can make all the difference. There's some good looking slopers on this boulder in Sweden (which also goes to show that sometimes, you just need to come back another day - persistence is everything!).