Here’s the definition of a Haka, courtesy of Wikipedia:
A haka (plural is the same as singular: 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.
War haka were originally performed by warriors before a battle, proclaiming their strength and prowess in order to intimidate the opposition, but haka are also performed for various reasons: for welcoming distinguished guests, or to acknowledge great achievements, occasions or funerals, and kapa haka performance groups are very common in schools.
This gives you some context for understanding the video :).
You can practically feel the emotion these boys poured into the Haka, even watching it on video. I can’t even imagine what it must’ve been like in person. Clearly Mr. Dawson meant a lot to them; I’m sure he’d feel honored.