It is short for Missing in Action. Which means the person (a soldier) disappeared in combat. However both the short form MIA and the term Missing in Action have also been extended to other things as in " He is MIA in the game of love", meaning the male has gone somewhere with his true love or just lost out (which meaning depends on context).

Yes, "MIA" means "missing in action". Basically, you don't know where that person is. Another acronym I teach with MIA is "AWOL" (ey-wol), which means "Absent Without (Official) Leave". The result is the same as "MIA" - the person is not there - but AWOL means the person should be there.