Backend workflows are similar to workflows that you create inside of a page or a component. The difference, is that these workflows will run on your backend or Server, instead in the frontend on users computer. (If the distinction between your frontend and backend is clear, check out "How the Web Works")

Because these workflows run in the backend, they can be triggered from anywhere in your app. Meaning, that instead of recreating the same workflow on multiple pages, you can often just build it once as backend workflow, and the trigger it where you need it.

Also, because it runs on your server, their actions, like sending emails and managing users, that aren't available in your frontend workflows for security reasons.