"Auld Lang Syne" is a poem written by Robert Burns in 1788 and set to the tune of a traditional folk song. It is well known in many English-speaking (and other) countries and is often sung to celebrate the start of the New Year at the stroke of midnight.
By extension, its use has also become common at funerals, graduations, and as a farewell or ending to other occasions.
The song's Scots title may be translated into English literally as "old long since", or more idiomatically, "long long ago", "days gone by" or "old times". Consequently "For auld lang syne", as it appears in the first line of the chorus, is loosely translated as "for (the sake of) old times".