Your display name is the name that represents you on Collectish. Unless you have changed it, your display name will be the same as the user id that you use to log in to Collectish.
Because user id's must be unique, they are rarely the same as your name, and they often don't look very friendly. Choosing a display name helps you to personalise yourself a little more.
What you use for a display name is up to you. Real names are fine, but you might prefer to use a nickname or some other word that suits you. If you're worried about maintaining your privacy, then we strongly suggest that you don't use your real name.