A gross-out film, or a gross-out scene, is made to shock you by being ugly to watch: vomit, toilets, blood, body waste, things bursting. It is not trying to be beautiful, and it is not quite the same as horror, which wants you to be afraid. A gross-out wants you to go “ugh,” and often to laugh at the same time. The word is informal and mostly American; you hear it around movies and comedy. A gross-out comedy is a whole film built on that kind of joke. You can also use gross-out as a noun for the disgusting bit itself. Though actually, the older piece is the verb: something grosses you out when it makes you feel sick. Gross here is slang for “disgusting,” not the school-book “total” or “12 dozen.” It is a casual, slightly teenage word. You would not drop it into a serious report unless you were naming the film type.