Foundational articles are long form content that solve a users problem, educates or shares your thoughts.
They are used to generate secondary content such as social posts and ideas for more posts.
Here are a few examples from my blog here:
1. What is the difference between a website, web app and mobile app?
2. How much does a web app cost?
3. What hosting platform should I deploy my frontend on?
Foundation articles do not need to be lengthy. All you need to do is clearly solve a visitors problem.