Conditions allow to apply some logic depending on other circumstances. 

There are 4 types of conditions you can use:

• Formula - use formula when you need to work with Source object, Variables, Current user, Environment, System data or Organization Settings.   
• Query - use query when you need to work with SF objects.
• Flow Step Status - build condition based on status of the specified process step and process instances.
• Call Apex - use call apex if you need to apply some complicated logic, which can't be applied with above options.

In places where conditions can be used, proper component can be displayed (e.g., in Actions configuration, 1). Select "With Conditions" option in the proper dialog  to add conditions. Then select condition type (2) and configure necessary conditions as described in the proper articles:

• Formula
• Query
• Flow Step Status
• Call Apex