Action is a configurable piece of user interface usually serving a single purpose. The configuration of actions is happening in the urlmap.yaml, for example:
/hello-world:
action:
type: page
title: Hello, world!
text: |
Hello, world!
The only required field to be configured for an action is type. It, basically, defines the behavior of the action and other allowed/required fields needed to be specified. title and icon fields are common for all actions. They are optional, but recommended for better user experience.