One of the main things you'll do on your FLOW account is create and work on stories. Part of this process is choosing the Assignment Types, or content/products, your staff will create for the story or package. Some examples would be Writing, Photography, and Video.
You FLOW account does come pre-loaded with several common Assignment Types, but you can customize the Assignment Types in your FLOW settings. You can also set Key Editors for each Assignment Type, which can be useful later on when creating Workflows.