The Expression Composer is a key element of the Bubble Application Editor that lets you build these expressions without typing anything, but instead by picking some options in some dropdown menus. The system will automatically prompt you in the next menu with options that are available given what you have selected so far. For instance, if the first main entry in the expression is 'Current User', the next menu will offer options such as 'is logged in', 'is not logged in', 'email', etc. In Bubble vocabulary, the main entry (i.e. the first choice in a dynamic expression) is known as a "data source" and all the chunks after it are known as "operators" (e.g. "email" in this case, or expressions like ":first", "contains", etc.).