Foundational content is the more static stuff on your website, like pages that explain who you are and what you do (like your about and services sections).
The problem with most foundational content is, let’s be honest, that it kind of sucks. It’s really focused on self-promotion (as it should be; it is, after all, your website) instead of being geared toward the needs of your customer.
The challenge with foundational content is to listen to your customer.