One of the biggest challenges in UI design is the gap between our understanding of the product, and our users' understanding.
We only know what we think we know: and sometimes not even that much. Though a user may be visiting your site for the first time, they'll know how basic elements work from their experience both on- and offline. The design of call to action (CTA) buttons comes from buttons we physically press in the "real world." Placing items in a digital shopping cart reminds us of choosing something to pay for later. Filling out an online form follows the same logic as a form filled out in pen, say, at the DMV or doctor's office.
Mental models are just what they sound like: a way of understanding how things work. When I see a door with no handle, I assume I need to either push it to open, or that it's automatic. When I select a button that's labelled Next, I expect there to more steps before I complete my task.
Sounds simple enough. But while good design borrows from known models, there will always be a disparity between the person creating something and the person using it. This shouldn't be surprising; the engineer of a car will know more about it than the person who drives it every day. When UX teams discuss mental models, the goal doesn't have to be equality: I don't want to know everything about how my car works. I just want it to do what I expect.
The danger comes when we expect a match. When you've been writing or designing for a new feature for weeks, you see patterns; you've collected a ton of data and know all about edge cases, logic, and maybe even how the developers are building it. We can't lose sight of the fact that our users aren't coming to the table with that same experience. When they see a screen for the first time, without all the extra data in your head, will it make sense? Careful UX writing can make that more likely.
tell a story - The order, structure, and flow of your copy should set the outline of a mental model that clarifies where we're headed.
be purposeful - It's important for us to know exactly how our product works, but what does the user want and need to know? Only provide info that is relevant and clarifying.
match existing models - Innovation has its place, but disrupting mental models that have been around for decades usually only leads to frustration. Don't label a field Cellular Numerals when we all know it's called Phone Number.
At every stage (brainstorming, designing, writing, revising), remind yourself of user expectations. Ask if you've provided the right amount of info for someone who's seeing your flow for the first time; ask someone with fresh eyes what they expect to happen. Mind the gap between our understanding.