Great question! Actions should be thought of as a mapping between some mechanical UI event (typing in a form, clicking on a button) and an event with some meaning in your application ("user typed a hacker news comment", "user submitted a reply to this comment with this text"). Whatever your application data store abstraction is, it should be responsible for taking that meaningful event and figuring out what to do with it (check to see that the message isn't empty, check to see that the comment was persisted to the server).

Actions describe things that happened in the world and the role of your store is to figure out if that thing that happened is valid or not, and pass the necessary data to components listening for changes.