You can think of a workflow as a plugin or extension to what RelaGit can do. A workflow can be anything from a script that runs a few commands when you commit your changes to a Discord RPC client that shows your current repo and file in Discord.
Workflows can do things like extend the settings menu, add context menu items, run scripts when certain events happen, and more. They can be as simple or as complex as you want them to be.
Workflows normally don’t require any setup. You can just install them and they will work. However, some workflows may require you to set up a configuration file or environment variable. You should be able to find the setup instructions in the workflow’s README file.