Sealed enclosures are the simplest of loudspeaker enclosures. They consist of a sealed cabinet, with the driver (typically) mounted facing outwards. usually the manufacturers specification sheet will list a suitable volume for a given driver. If the enclosure is too small, the driver may be damaged by the high pressure that builds inside the box, or even damage the enclosure itself.
Sealed enclosures are the closest realistic approximation of an infinite baffle, and as such are sometimes called infinite baffle enclosures.