The Expression Task creates and evaluates expressions that set variable values at runtime, using the Expression Builder. When you edit the task, the Expression Builder is launched.
The expression language includes functions and operators. For examples that demonstrate how to use the functions and operators, see the Expression Examples sections in the functions and operators topics. Links to the topics are in Functions (SSIS Expression)and Operators (SSIS Expression).
For examples of more complex expressions, see Examples of Advanced Integration Services Expressions.
For examples of using expressions to set properties, see the Sample Property Expressions section in Use Property Expressions in Packages.
Technical article, SSIS Expression Cheat Sheet, on pragmaticworks.com