How does prism armor work?

Question

I mean, does it make it so that its like it has no weaknesses, or what

Answer 1

Prism Armour
In-Depth Effect:
Reduces the power of Super Effective moves used against this Pokémon by 25%

So it makes all weaknesses, that would normally deal 1.5x damage, deal 1.125x damage. (1.5*0.75=1.125)

Source