It can simply be used to describe a border between two places. However, it usually has the meaning of a border which separates that which is known from that which is unknown. Georgraphically, this could describe mapped from unmapped land. Or land which is 'ours' and known, from land which is 'theirs' and not known. Or it could be used to describe research which has 'gone beyond existing frontiers' and has found itself at new frontiers. The American "Wild West" was known as frontierland as people explored westwards in the 19th century.