Flow
A flow is an application that can execute logic, interact with the Salesforce database, call Apex classes, and collect data from users. You can build flows by using Flow Builder.
For more info on creating, flows refer to How To Create A Flow?
Flow Actions
A flow action is a Process Builder that launches a flow. With flow actions, you can automate complex business processes, create flows to perform logic, and have events trigger the flows via process builder—without writing code.
One thing you need to remember that you can only automate autolaunched flows using process builders.
Flow Builder with Flow Example

Process Builder Example

Also Read: How To Call Flows From Apex?
Criteria to Execute the Flow

Selected Flow in action to execute the flow

Here I am setting up the value of Flow variable

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