The best definition I could find online is as follows:
The "Maori sidestep" was first used by the New Zealanders, and occurs when a player doesn't try to avoid the tackler, but charges him head-on, bumps off the defender, and generally tramples him as he runs over the top.
A couple of warnings, however - the first regarding usage, the second regarding the manoeuvre itself.
On the topic 'Rugby clichés you would like to hear', one member of The Silver Fern.com forums comments: "Maybe im wrong here but i don't think [the term 'Māori sidestep' is] culturally insensitive or racist. it's kinda cool."
That suggests, of course, that the term is sometimes perceived to be "culturally insensitive or racist"...
And here are a couple of extracts from a letter published in Volume 296 of the British Medical Journal:
The Maori side step, known hereabouts as "bursting the tackle," is a highly dangerous manoeuvre where the runner aims himself directly at his tackler.
[It] is more dangerous than the high tackle and should be banned from schoolboy rugby immediately. Adult rugger could also do without it.
You have been warned!