After registration everything is automatically setup for you
A new brewery: This will hold everything "brewing" related (recipes, hardware, etc.). You can also add others to the brewery. Read more here: Managing your brewery.
Your profile for profile picture, username (to make it easy for others to add you to their brewery).
Most importantly: Your settings. These allow you to customize, e.g. all units used in the UI to your needs. Read more here: Your settings.