A tack is a small, sharp pin or nail with a wide head. You can use tacks to attach your flyers to the telephone poles in your neighborhood.
As a verb, tack means "attach using a tack," and also, simply "add," the way a movie director sometimes tacks on a happy ending to satisfy the audience. In sailing, a tack is a change of course that results in the wind blowing on the opposite side of the boat: "We decided to make a sharp tack to avoid the iceberg." Tack can also refer to a certain rope used in sailing, or to horseback riding equipment.