When you write an expression, you have access not only to Global Objects, Functions and Aggregate Functions but you can also access a member of an object, using the . (dot) character. You can call all public members of an object directly in the Expression Editor. Here are a few examples:
= Now().ToShortDateString(): Returns a string that contains the current date in short format.
= 'abc'.ToUpper(): Returns "ABC" string.
= 'abc'.Length: Returns the length of the current string, 3.
= Price.ToString('$0.'): Returns the value of Price formatted according to the specified format, i.e. $123