How do unpolarized sunglasses reduce brightness?

Hi everyone!

I've been reading up on light and transmission, absorption and reflection and I was wondering how unpolarized sunglasses are able to reduce brightness. I tried Google-ing this but all I found was information about polarized sunglasses and how they reduce glare and work as UV protection but I have a fairly solid understanding of that. How do basic sunglasses work? Do they reflect some frequencies of light but let others through? If so, which ones?

Thanks for all your help!