The term down under is a colloquialism referring to things related to, or coming from Australia (sometimes known as 'the land down under').
"Down Under" is also the title of a song written by the Australian rock group Men at Work which has become a popular patriotic song by virtue of the popular interpretation of the lyrics being about an Australian traveller circling the globe, proud of his nationality and attracting the interest of others because of that. This song references vegemite on a sandwich, among other things.
The most well known part of the lyrics is the refrain:
Do you come from a land down under? Where women glow and men plunder? Can't you hear, can't you hear the thunder? You better run, you better take cover.
See also Aussie