Invocable processes are the processes which are built to modularise the code and reuse it whenever required, for example when your organisation closes won an opportunity, then a follow up task created for the owner to touch base with the account contact. Similarly when an Opportunity is closed Lost, then a follow up task is created to touch base with the Account contact. 

Therefore it makes sense that you build a process build which creates a follow up task and is then called from different nodes of another process and gets executed. 

To Create a invocable process you need to follow the below process. 

1. Click on the Gear Icon on the right hand top corner. 
2. Type Process Builder in the Quick find box. 
3. Click on the process builder. 
4. Click on the new button and fill out the below fields. 
   1. Process name: Opportunity Follow up Task
   2. API Name: Opportunity_Follow_up_Task
   3. The process starts when: It’s invoked by another process

5. Click on the object and since we want the task to be associated to an opportunity, we select opportunity as the object and click on save.
6. Click on the Criteria node and Select: No criteria—just execute the actions!

7. Save the Criteria and click on the ‘+ Add Action’ button and define the Action as below
   1. Action Name: Follow up task
   2. Record Type: Task
   3. Set Field Values:
      1. Field: Assigned To ID , Type: Field Reference, Value: [Opportunity].OwnerId
      2. Field: Priority, Type: Picklist, Value: High
      3. Field: Status, Type: Picklist, Value: High
8. Save the action and Activate the Process to use in other processes. 

To Call this process from another process, we will use the existing process ‘Opportunity Won’ 

1. Click on the process Opportunity won and click on the clone button to create a new version of this process. 
2. Click on the ‘+ Add Action’ next to the node Opportunity won and define the Action as below.
   1. Action Name: Follow up task
   2. Process: Opportunity Follow up Task
   3. Set Process Variables
      1. Process Variable*: SObject
      2. Type: Field Reference
      3. Value: [Opportunity]

3. Save the action and repeat for the Node Opportunity Closed Lost 

The overall Process should look like this. 

4. Click on save and Activate the process. 

For More Examples refer : Automate you Business Process