When you cut something, literally or figuratively, you trim it. If you have steady hands, you can trim your own bangs. If you don't have steady hands, you can save up for a professional haircut by trimming the amount of money you spend on comics and snacks.
A landscaper trims the grass and shrubs, and a cook trims fat off of meat before cooking it. But when you trim the Christmas tree, you decorate it, and when a sailor trims the sails, she adjusts them so they'll catch the wind just right. And as a noun, a trim is "an act of cutting," while trim is "decoration." All forms of this word are rooted in nautical language.