This tab displays Checklist containing Items - as configured for step at Process Builder, as added to this step additionally.

The Checklist represents a sequence of simple tasks that should be done for some purposes (in many systems they are called Standard Operational Procedures). Checklist items can be checked or not; if at least one of them was changed, checklist became needed to be saved.

Checklist Items can be required - they are marked with proper sign (1). Such items must be checked to allow step to be proceeded (but do not prevent checklist from being saved); if try to proceed a step, which checklist has non-checked required items, a proper error message will be shown (2).

User can add own checklist items to the checklist by clicking "Add" button (3). The proper small form will be shown (4) - populate checklist item label (5), check "required" checkbox, if necessary (6) and click "Apply" button (7).  

These additional checklist items can be deleted from step in this tab: click the proper icon (8) and click "Yes" button in confirmation popup (9) that will appear: