I am a new user on Stack Overflow. When I look at my profile, I see that I have got two accounts, which are "Stack Overflow" and "Super User." Have I done something wrong? What do these accounts mean?
You have two user accounts:
This is not a problem at all, you didn't do anything wrong. If either account reaches 200 points or more, you'll receive an extra 100 points on your other sites because you associated the them.
The two accounts are linked. You can add many more if you join any of the other sites in the Stack Exchange network of sites.
The SuperUser account is your account from https://superuser.com/ (it also stack exchange).