Our organization created its own expression language, based on JavaScript Expression Language (JEXL). Users can create powerful automation to customize an application by writing expressions but client system administrators are not necessarily technical or experienced programmers. The Expression Language knowledge base article teaches users how to write expressions and documents all the functions they can use in them. I break down the fundamental concepts of expression language in this article, using labelled diagrams to illustrate expression language format and structuring the content in accessible sections. Later article sections include a glossary of all functions, which users can reference after they learn how to write expressions, and example expressions they can paste into their application based on common client use cases.