When you join Harry Potter and the Sorcerer's Stone™ you will have the opportunity to select your Hogwarts™ House. You can also choose to skip this step and and revisit it later.
If you need help choosing your house, there is a Sorting Hat website which you will find linked within the App.
Choosing a house in the App is a one time option and you cannot change this house for the duration of the challenge series.
If you do select a house you App experience will have some elements that are themed to match your house. If you don't choose a house you will either not see this themed element or it will be Hogwarts™ themed.
Choosing a house also means that your house points that you earn will be assigned to your house.
On the map your house animal will display above your avatar. If you have not selected a house then the symbol above your avatar will be an H to represent Hogwarts.