You can configure Dynatrace to identify you users. Identification can be done by getting a value from a meta tag, a css selectore, a cookie or a global JS variable. You can use user id's or things like team name or some other grouping identifier you want to use. The user tagging can be configured per web application in the application settings.
Anonymous means that there was no user tagging found. The long id is a unique generated identifer stored in a persistant cookie to identify returning user again.