A mine is a payload of ordinance, typically explosive, that can be laid in space, usually in large amounts. When used to defend areas of space, mines can be more described as simple drones, due to the fact that they have to move towards nearby targets to even have a chance of exploding. Mines will typically have cheap components like a simple radar and anti-gravity drive in order to detect targets and attempt to intercept them, and some mines may have more sophisticated components that allow for features such being unable to target friendly ships.
Where mines shine, however, are when placed into hyperlanes. Due to the narrow diameter of most hyperlanes, ships passing through a minefield in a hyperlane are often at considerable danger. Mines, unless in large amounts, can be defended against using point defense systems.