An expression is an instance of a function or a reference to a map element used to define a transformation. Whereas a function defines an action (like adding two numbers), an expression is the use of that function as part of defining the value for an output map element, for example.
Expressions are arranged in trees where the result of each expression becomes an argument value for the parent expression. Expression trees are used to define the values for output map elements. The root expression of the output value expression tree is the value of the output map element.