When you choose to use an Expression condition you define a text expression that returns a boolean value. The value returned determines whether or not the formatting is applied. The configuration panel is shown below:
Field Is. Field is used to select the type of conditions.
Expression. This field is used to define an expression that should return a boolean value.
For example, a suitable expression in C#:
Customers.CustomerName == "MyCustomer"
If the expression cannot return a boolean value then the report generator will not be able to render the conditional formatting.
Important: The expression MUST return a boolean value or the conditional formatting will fail.