You may hear our team talk about semantic layers or meta-data layers on our videos, specifically when talking about the DashboardFox App and App Building.
The short answer, a semantic layer translates the technical details of your raw data into something that is easily understood and re-useable by your Composers and Viewers.
Instead of having to hard code and write SQL code against a database, you now have a "Semantic Layer" or a DashboardFox App that you can simply pick what you want and have all the SQL code generated automatically.
We wrote an article on our blog going into more details that you can read here - https://dashboardfox.com/blog/what-is-a-semantic-layer-and-what-does-it-do/
And our Getting Started Guide has an overview of the App Building process here - https://help.dashboardfox.com/article/show/107974-configure-your-dashboardfox-app