Something that's exterior is on the outside of some structure or object. An exterior door on a house opens to the outdoors, rather than connecting one room to another.
Exterior house paint is made for painting the shingles or siding of your home, not the walls of your living room, and the exterior finish of a car is on the outside. You can also use exterior as a noun, to mean the outside of something: "The exterior of this salad bowl is a weird shade of yellow." Exterior is a Latin word, a comparative of exterus, "on the outside, outward, or foreign."