A variable is a named value you can read inside any expression. In almost all cases, variables come from input fields: each field creates one under its variable name, holding whatever the user enters.
To use a variable, write its name in an expression. A field with the variable name brandColor lets you read brandColor wherever you need that colour. Because several elements can read the same variable, they all update together whenever the user changes that one field.
In the expression editor, the Variables tab lists every variable available to the element you are editing.