Polarising cuts out some of the light by only allowing it to pass through in one plane. They’re good for fishing and sailing as the surface of water has the same effect, so by polarising the glasses at 90deg no light gets through, meaning you can see through the surface and see the fish. Same for sailing, they show up ripples on the water indicating a breeze.
If you use anything with an old LCD display they work on the same principal, passing an electric current through one layer aligns the crystals, polarising the light, the top layer is fixed, so when both layers are polarised in the same way they’re transparent, when you put the charge at 90deg it goes black as the light cant pass through both layers. That’s why polarised glasses mean you cant see the screens, but if you turn the screen through 90deg the fixed layer on the screen and the glasses will line up and it’ll be visible.
Not much use for MTB though, no need.