An antimatter mine is a relatively small device only the size of a small probe that contains a large amounts of antimatter. Onboard sensors detect close-range starships and the mine moves towards them. On impact the containment fields within the mine collapse dispersing antimatter onto the ship's hull causing a large number of instant annihilations resulting in an explosion.
In 2369 the crew of the USS Enterprise-D used 500 antimatter mines according to orders by Captain Edward Jellico to force a cardassian fleet hidden into the McAllister C-5 Nebula to withdraw. The mines were placed surreptitiously by a shuttle and tacked themselves on the hull of the cardassian ships. (TNG: "Chain of Command, Part II")