Rock no longer weak to Steel, Water no longer resistant to Steel:
One less weakness for one of the worst types defensively, one less resistance for a type that has 4 but only 2 weaknesses.
Steel no longer resistant to Dragon, Ice to resist Dragon instead:
One less resistance for the most broken type (not as absurd as in before Gen 6 but still OP). Ice needs that resistance way more, and it makes sense since they're supposed to be good Dragon counters.
Dragon no longer resistant to Grass, Fairy no longer resistant to Bug:
A little boost to fix Grass and Bug moves being resisted by seven types, while nerfing Fairy a little defensively.
Ice resists Ground:
Resisting Ground, and Dragon, Ice would stop being so garbage defensively. And since Ground is one of the best types offensively, and Earthquake one of the best existing moves, it could use a little nerf.
So, boosted types: Rock, Ice
Nerfed types: Steel, Water, Fairy, Dragon
Boosted moves: Grass, Bug
Nerfed moves: Ground