The haka is a traditional ancestral war cry, dance, or challenge from the Māori people of New Zealand. It is a posture dance performed by a group, with vigorous movements and stamping of the feet with rhythmically shouted accompaniment. The New Zealand sports teams' practice of performing a haka before their international matches has made the haka more widely known around the world. This tradition began with the 1888–89 New Zealand Native football team tour and has been carried on by the New Zealand rugby team since 1905. Here is the background: https://en.wikipedia.org/wiki/Haka

But it's best to see it. Here are the NZ All Blacks doing their stuff before the game against Argentina during the 2015 Rugby World Cup:

https://www.youtube.com/watch?v=1ujq9sXanYM

More Info: en.wikipedia.org