Rules are special conditions that prevent step from being proceeded.

Go to Process builder. Open Step Settings (1) and expand "Rules" (2).

Click on "add" link (3) to open popup (4).

General configurations of step rule are:

• Name - used just for comfort and does not affect rule execution
• Status - rules can be active (working) or inactive (e.g., temporary disabled).  
• Error Message - user must enter the error message. It will be shown if the rule matches defined conditions (5).
• Execute Before Step - if  checked, the rule will be automatically applied before step become active and, if rule matched, step will get status "Pending". Error message will be shown as Status Reason in the proper step tab (6).

Rule, if added, must have conditions configured and they must return boolean result (true or false).

Conditions can have different types:

• Formula - logical expression with boolean result
• Query takes value from specified object field. If more then one record matches query conditions, the first value will be taken
• Flow Step Status simply checks defined step(s) for one or several statuses. User can select any step from any process
• Call Apex - executes specified Apex Class with given parameters and returns the result. Result should have boolean type

Save configured rule. Rules will be shown in step properties. They can be deleted (7),  added to library (8), or edited by clicking rule name (9).

"Clone" button (11) will appear when the rule is being edited. After clicking it system will ask for cloned rule name and target step (12, mandatory).

After clicking "Apply" button an edit form for cloned rule will be shown.