User!UserID is a standard fields available within a report. Reporting services gets the value from Windows. It contains the windows/AD user name of the user who is running the report. So, if you domain name is "mycompany" and you login as user "myuser" then the value of User!UserID will be "mycompany\myuser"

right(User!UserID,10)" gives you that last 10 characters of the variable User!UserID. In my example, this would be "any\myuser". I suspect that you company has user names that are 10 characters long and the "RIGHT" function for you would be returning the person's user name