Hi Sumit,

1.  A record Changes:
As its name reflect, the process would start when the selected object record get changed.
2.  It's Invoked by another process:
When you create a process, have it start when another process invokes it by selecting It’s invoked by another process.

Consider following point before selecting "Invoked By another process":
1. You can invoke processes with objects that share at least one unique ID
Example:
Consider the Account and Case objects, the AccountId field is unique to Account
and also used by Case. You can create an invocable process that update a Case
record. Then you can invoke it from:
     1. A process that updates an Account record's Owner
     2. A process that adds an Account shipping address or updates it
2. When you create a process that invokes another process, each one counts toward your process and other applicable limits. DML limits in processes that invoke processes count as one transaction.
3. When you add an object, pay attention to the object you select. Your selection determines which other processes can invoke the one you’re creating. Only processes based on objects that share at least one unique field ID can invoke or be invoked by another process.
4. When you define or edit your process actions, select Processes. Only active invocable processes are available.
Benefits:
With invocable processes, you can reuse sections of your processes. Build one invocable process, call it from multiple processes or from multiple action groups in the same process. This ability to reuse can save you precious time.
View Your Process Types on your list view of processes:
The new column has been added to list view of processes as "PROCESS TYPE"
Values:
1. invocable :- Process can triggered from another process
2. BLANK :- Process can not called by another process.

3. A platform event occurs
Please refer the below link:

https://releasenotes.docs.salesforce.com/en-us/winter18/release-notes/rn_forcecom_process_events.htm

Hope This will help you.
If this answers your query please mark this question as a solved so that it can be filtered out from unsolved questions.

Thanks,
Jainam Contractor,
Salesforce Consultant
Varasi LLC
www.varasi.com