This article explains the basic concept behind a Frontend Server Architecture in a short paragraph.
Goal of architecture
Rebalance processing pressure in the presentation layer from light-weight Frontend clients towards more powerful server machines in a layered fashion. Divide and conquer (of responsibilities) is the basic approach behind the concept.
In a distributed architecture there is often the need to decouple complex algorithms, that were placed in the presentation layer from light-weight Frontend implementation like a Browser-Environment (or smart device), towards a usually much more powerful server machine.
In a Frontend Server Architecture there are actually three server layers.
The purpose of the Frontend Server is to serve presentation services with transient data handling. The Backend Server is the classical backbone to the business methods to fetch and process persistent data, whereas the database management system (DBMS) serves the persistent data towards the Backend Server.