View the original community article here
Last tested: Feb 12, 2021
In short, the action hub is the server that processes data actions => https://docs.looker.com/admin-options/platform/actions
Action hub is a full framework for managing api keys, integration settings and a few other things. There's an action hub server that sits between Looker and the integration (e.g., twillio). You can additionally set up an action like this: https://docs.looker.com/reference/field-params/action where you are pushing data directly to your integration or some other middleware that you've configured yourself. Action Hub is more secure because you don't have to store your API credentials in plain sight. That said, it is much harder to configure if there's no pre-built configuration.