A mine is an explosive device used to make a particular area impassable to enemy troops or vehicles. It consists mainly of an explosive charge connected to a trigger mechanism; a housing designed to either fragment or direct the explosive force is also typical. Various trigger mechanisms can be used, from fairly simple pressure-based systems to sophisticated sensors. When properly triggered, the mine explodes, damaging whatever triggered it.
Historically, mines have been deployed on land against troops and armoured vehicles and at sea against surface ships and submarines.
Mines in Science Fiction
Mines have made repeated appearances in science fiction in the form of "space mines", despite their relative uselessness in a large 3D space; a spaceborne minefield can easily be circumnavigated. The logical solution would be to use missiles deployed in likely traffic areas that would activate when a spacecraft got within range.
One notable use in the Star Trek universe is the minefield used to deny the Dominion access to the Bajoran Wormhole. This space minefield is actually plausible, since it surrounded a single point in space rather than trying to block a border or otherwise saturate a large volume of space with mines. Similarly, mines were once deployed in the "Hekaras Corridor", a space-travel bottleneck formed by navigational hazards.
During the Dominion War, the Dominion used a specialised type of anti-personnel mine that would spend most of its time in subspace, only emerging periodically to scan for targets and detonate. This mine was not very effective, as it did not deny the enemy passage through the mined area, merely causing occasional casualties. The Federation made more effective use of the mines after commandeering them, saturating a narrow valley with mines to cause heavy casualties to Dominion forces.
In the B5 episode "Matters of Honor", we see a Centauri minefield deployed to interdict access to a planet. This minefield actually consists of orbiting weapon platforms that fire on intruding starships instead of merely exploding when they get within range.
Captain Sheridan also used nuclear warheads as mines on at least two occasions, but these were remotely detonated from a starship when enemy vessels were deemed close enough. On one of the two occasions, a radio signal was used to lure the enemy ships into range; on the other, the mines were distributed around a planet known to be the target of enemy attack, and only a handful ended up being in range.