Smut in this context reeals explicit sexual content which might not be suitable for everyone. To illustrate, focussing on the physical intimacy between characters, a coming-of-age like 'Nine and a Half Weeks' could scarcely be called moral. However, 'smut' doesn't imply sensitivity, vulgarity actually. Smut is just a way of referring to books with mature sexual themes which aren't suitable for children or reserved adults. Smut, in other words, could have its own genre or be a part of another like romance or fantasy. Still, depending on the author the level of explicitness varies remarkably. Some want detailed, unbridled accounts and then you have authors who prefer a more implied, subtle approach.