Rather than doing my own detailed Waitangi Day post, I’ll recommend Peter Cresswell’s post, as he has written it far better than I could (bar his proposal of a new constitution, which I believe is unnecessary).
If you really want to know what Waitangi Day is about, read the treaty. Too many people go on about racist concepts they have invented, ignoring that the Treaty actually says:
In return for the cession of the Sovreignty to the Queen, the people of New Zealand shall be protected by the Queen of England and the rights and privileges of British subjects will be granted to them.
The Maori version is even more explicit
Hei wakaritenga mai hoki tenei mo te wakaaetanga ki te Kawanatanga o te Kuini-Ka tiakina e te Kuini o Ingarani nga tangata Maori; katoa o Nu Tirani ka tukua ki a ratou nga tikanga katoa rite tahi ki ana mea ki nga tangata o Ingarani.
Literally: This is an arrangement for the consent to the Government of the Queen. The Queen of England will protect all the Maoris of New Zealand. All the rights will be given to them the same as her doings to the people of England.
The treaty established one law for all. That is what we should be remembering and celebrating today, the end of tribal warfare and slavery, and the institution of equal rights for all – the legal framework that has given us the country we enjoy today.
EDIT: See also my post on flying a Maori flag on Waitangi day