Once you've completed a course, you will automatically receive a Certificate of Completion as proof of your accomplishment. If you have not entered your first and last name on the Account Details page, though, then your certificate by default will display a string of numbers and letters (as shown below).
In order to have your name displayed correctly, follow the instructions in this article: How can I change the name displayed for my account?
After changing your name, your certificate will now display the first and last name you entered. This affects both old and new certificates, but you can change the name displayed at any time.