If you almost never do something, you can say you do it rarely. If you walk or bike to work most days, you could say that you rarely take the bus.
Something that happens rarely happens only once in a while, or seldom. You might sigh that your town's baseball team rarely wins a game, or boast that you rarely forget a person's name once you've been introduced. The word comes from the adjective rare, "not occurring often," or "unusual," from the Old French rere, "sparse," and its root, the Latin word rarus, "thinly sown, with intervals between, or full of empty spaces."