Put simply, a Mental Model is "the picture I have in my head of how I expect this to work."
In relation to the Information Architecture framework it goes like this:
A user arrives at your website and gets presented with a whole lot of information
They then need to process this information
They can then make a decision.
Inside of the processing is the Mental Model, and it includes pre-existing expectations.
Expectations generally come from past experiences a user has had on other websites, namely “what am I looking for, and how I will find it.”
If I am on an e-commerce website I may be looking for:
a search bar
how to sort the products by price or reviews
where the contact details are, etc.
For me I’ll look in the footer, or up in the top right hand side for a phone number – everyone is different, and there are industry norms that the user expects too.
It’s really important to consider the mental model a user may have before visiting your website so you can plan your content structure and user journey accordingly.