Flow Actions get installed into the palette of Flow Builder, and you can drag them into your flows and hook them up. Each expects certain inputs from 'upstream' in the flow and makes outputs available.
Local Flow Actions are built with Lightning Components and have some special characteristics.
A new Introduction to Invocable Actions (https://medium.com/@alexedelstein/understanding-invocable-actions-af423cca4b)
New Get Random Number Flow Action - No more workarounds! (https://medium.com/@alexedelstein/easy-random-numbers-come-to-flow-15de26fa712f)
Deep Dive on Building a Flow Action in Apex (https://medium.com/@alexedelstein/how-to-build-a-flow-action-with-apex-38e2210c0d93)