This illustration shows the Expression Editor dialog box. 

The Expression Mode drop-down list is displayed at the top with a value of Simple. 

The Expression Editor is then divided into these areas, from top to bottom: 
Expression field (free-form text field with a value of taskOutcomeDataObject =="REJECT")
Insert into Expression and Validate (buttons)
Data Objects (selected) and Operators tabs
Description and Errors tabs

The Data Objects and Operators tabs let you select data objects and operators for your expression. 

The Description field (selected) displays the description of the selected operator.

The following items are bordered in red, to call attention to them:
Expression field
OK button