Actions in Flow (Invocable Actions)
Flow Actions, also known as Invocable Actions, are a powerful, easy-to-use way to extend Flow, Next Best Action and Bots. Invocable Actions can also be called programmatically by Apex code, giving developers a way to easily generate reusable code modules. Actions can be invoked by REST directly, and can be written in Javascript to integrate local, browser-based functionality.

Install Actions Into Your Org
You can find a wide range of actions here that can be added to your org to extend its capabilities. You can also find actions on AppExchange and write them yourself.
USING INVOCABLE ACTIONS
Introduction to Invocable Actions
Learning about “Local” Flow Actions
Available Actions You Can Install
Using Invocable Actions with Einstein Next Best Action
Using Invocable Actions to Manipulate Web JSON REST Responses
Using Invocable Actions to Add Advanced SOQL to Flows and Recommendation Strategies
Using Invocable Actions to Integrate Quip with Salesforce
BUILDING AND PACKAGING INVOCABLE ACTIONS
Building and Packaging Invocable Actions
Calling Invocable Actions from Apex
Adding Custom Icons to your Invocable Actions
Using Apex-Defined Types (aka Custom Types)