A screen ID is needed to know what content to show your screen, whether it is one of products or a digital display that you may already have. It allows you to manage your screen remotely and once it's setup, your screen will receive real time updates for new, edited, expired or deleted posts.
You can find out more about using HootBoard on screens here. In order to create a HootBoard Screen ID, you will need a Gold Plan or better. Here's a screenshot of how you can create a Screen Id.