GitHub Actions is a CI/CD platform built into GitHub, made by GitHub, Inc. CI/CD means continuous integration and continuous delivery: every code change is built and tested automatically, and accepted changes are deployed automatically. Actions also automates other repository tasks, for example adding labels to each new issue.
A workflow is a YAML file in the .github/workflows folder of a repository. It runs when an event happens, such as a push or a new pull request, on a schedule, through the REST API or by hand. A workflow contains jobs; each job runs on a runner, a virtual machine or container, and executes its steps in order. A step is a shell script or an action, a reusable extension, often taken from the Actions marketplace. GitHub provides Linux, Windows and macOS runners; teams can also run self-hosted runners. A matrix runs the same job on several operating systems or language versions, and encrypted secrets keep keys out of the code.
Actions is free for public repositories and on self-hosted runners. For private repositories, each plan includes free minutes; extra minutes on GitHub-hosted runners are billed per minute.