This property on the variable seems to be causing a bit of confusion. What it basically does is says that the value of the variable is not going to be a literal value that you enter rather the value will be the result of an expression. This is consistent with everything else that uses expressions

Let's look at an example

Here we have said that the value of the NameOfFileFound variable will be evaluated as an expression. The value it will take is whatever is the result of this expression

 DATEADD("yy",2,@Variable)

The difference with the expression editor here and everywhere else is that here you get no UI. My way of circumventing this is to use an expression editor from another task and then copy and paste it in.