A sex blanket is a blanket that's literally made for sex! It's made with a waterproof, leak-proof core that stops liquid from leaking though and ruining your bedding and mattress. It should be large enough to cover the top of your bed -- no need for it to drape down over the sides like a bedspread or comforter because, well, there's nothing happening on the sides of your mattress.
The best ones are made from super velvety micro fleece so they feel great against bare skin. And micro fleece is kinda grippy, so it generally won't shift around during use.
Some sex blankets can feel stiff and sound crinkly or diaper-like -- that's because the waterproof core is too thick for the weight of the fleece. A well made waterproof sex blanket will be pliable and virtually noiseless.