“Auld Lang Syne” is a Scottish poem set to the tune of a traditional folk song. It’s well known in many countries, especially (but far from exclusively) in the English-speaking world. Its traditional use is to celebrate the start of the New Year at the stroke of midnight. This performance is by Dougie MacLean.
Many people sing it to bring in the new year without actually understanding its meaning or even the words. The text on which the lyrics are based isn’t in English at all — it’s 18th-century Scots, a similar but distinct language. The words had been used by a Scottish church (S. w. Ayreshire) for the previous 600 years, as a funeral oration, and was finally documented by Robert Burns in 1788 and included some original content written by him.
There are several translations, which you can see here on Wikipedia along with more background info. In the mean time, enjoy the song and have a Happy New Year!