A GitHub Action is a script which allows you to automate your GitHub workflows.
Specifically, an Action is an action.yml file paired with a Node script (found in dist/index.js). The action.yml file defines the inputs, outputs, and other metadata used by the Action, which the user can control in their own project. GitHub will execute your Node script with the parameters from the action.yml file after various events such as pushing to a branch or creating a pull request. Once you publish a GitHub Action, it can be used in other repositories to automate their workflows.
GitHub's Actions Toolkit is a set of Node modules which provide useful tools for creating Actions. This library provides bindings to the packages within the toolkit in order to define GitHub Actions in PureScript.