Ask HN: How to market a product before launch?

As someone new to entrepreneurship, I would say 90+% of the advice I've been receiving so far is 'make sure people want your product before you build it', or various platitudes along those lines.

We're well on our way towards a polished 'beta' launch of our product, but I took this advice somewhat seriously and went ahead and developed some pre-launch marketing materials for our product that essentially give a preview of the UI and describe our various features.

However, I've realized that I have no clue what to do with these. How am I supposed to get people to care about some images and text that describe a product that they can't use? Where would I even post such materials?

More generally, I suppose, how is one supposed to test product-market fit without a product? I'm really not sure how I should be interpreting and acting upon this advice.