In short, the user with the UID (user ID) who created your tar file doesn't exit on your system.
The user ID is the third column.
Suppose I were to create a .tar file containing files created originally by "user1", and give you that file. Now you extract that file, but you have no user with the same user id (9001). As a result, the files will show in a listing as owned by user id "9001" and not a name.
You can fix this with chown, as root (group is optional):
chown -R user.group