As I understand it that would give a 'hard' border between NI and Eire, leading to customs posts and all the rest of it.
Which is what quite a lot of the brexit wrangling has been about trying to avoid, because that goes against UK/Eire agreements such as the Good Friday agreement.
Instead the brexit deal made the Irish sea a kind of 'rubbery' border, neither one thing or the other, the UK could invoke art.16 to change that which would them mean a hard border between NI/Eire.
It may only be partial/temporary under article 16.
But as I said above trade wars have started over less.
It could even end up with the whole brexit agreement being torn up. (ie. No deal brexit).
The various news agencies are catching up on what it means and publishing articles now; have a google for 'article 16 of the northern ireland protocol'.