To learn more about how to use Workflows, check out Understanding Workflows.
Within the Workflows settings, you can choose which Workflows are added to Items of your Module on creation. Adding Default Workflows can be helpful when you have a set of repeatable processes associated with your Module.
For example, if you created a Module for Vendors, you may have multiple workflows associated with onboarding your Vendor, End of Year Vendor assessment, and submitting Vendor invoices. Instead of asking or trusting members of each individual Vendor to find and execute the right workflows every time they onboard a new Vendor, you could attach these three Vendor workflows as Default Workflows.