We learned that the colours on the labels are like the colours on the traffic lights. The M rating means that although anyone could see the movie, it is more suited to older viewers.
The classification office of New Zealand says:
"The M classification means the movie might contain violence, offensive language, drug use, sexual or adult themes or nudity that some kids and parents find challenging.
A lot of blockbusters based on books popular with the 10+ age group are classified M, including most of the Harry Potter movies, The Hobbit trilogy and The Hunger Games."
Here is a link to the Classification Office website for more information:
https://www.classificationoffice.govt.nz/blog/m-label/