We're glad you asked! It's certainly a bit confusing.
When you sign up with Cisco.com, you're asked to create a User ID, which you can then use to sign in (or you can use your email address). On the Cisco.com registration form, this is referred to as a User ID, and on the login form, it's referred to as a User Name.
Your Community Display Name is the name that others will see for you on the NSO Developer Hub. You set it up the first time you enter the community. It can have upper & lowercase letters, underscores, dashes, and numbers. You could choose to have it the same as your User ID / Username, or it can be different.
Why is there a community display name at all?
The name you use on the community is pretty visible, so we wanted to make sure you had a chance to choose it knowing it will be public. This is particularly important for Cisco employees, who don't get to choose their User IDs. Some of the IDs are pretty unfriendly / not reflective of how the employee wants to be known publicly. I definitely prefer "Nicklas Wagerth" vs "nwagerth" as my identity here in the community!
