Very simple, really. It changes the helmet textures to match the in-game models a lot better.
The vanilla textures of Helmets, while iconic, don't correlate with how the armour looks when worn at all. This resource pack fixes that by giving them the new square shape.
Each helmet has its own shape, too. Such as chain mail having it being on all sides, and the Turtle shell having the small brown straps.

Even the trims from 1.20 have now been updated to this new shape, that way it doesn't look off.

So... yeah. Its pretty simple, really. Go use it!