Within the editor you may see "flask" buttons . These buttons will open the Expression Builder.
The Expression Builder has 3 sections:
1 (Red): Pre-made expressions you can use for reference or click the Copy button to copy the expression to the editing area.
2 (Blue): Editing area where you can type the expression
3 (Orange): Verify button used to check that your expression is correct
4 (Green): Test button used to open the Expression Tester (more info below)
5 (Pink): OK button to save changes and close the expression builder
The Expression tester is used to quickly preview what the expression result is for each row.
In addition to the expression result, you can expand the expression to see what each step of the expression does.