This is where you insert your Magento Keys (API that links your store to the Magento Marketplace accounts). They are the same Keys you would have used when you installed Magento.
However, if you didn’t install Magento yourself then you can learn how to get those keys by watching this (How to install Magento 2.3 and build a web server) just after the 38 minute mark. It takes about 2 minutes.