"Minlock" refers to an area being locked from players who have not yet
reached a certain level of experience.  That is, you have to be of a
certain level or higher before you are able to access the area.  A
character who is below the level of the minlock is said to be 'minlocked'
and is unable to enter the area until they achieve the required level. 
Areas are minlocked for a reason, with some being very dangerous.  Beware
however, not all dangerous areas are minlocked - many a young explorer has
perished whilst traversing the Retroverse.

See Also: help explore, help level locked, help maxlock