The GitHub App is the new way that Morph gets access to GitHub and does things on your behalf.
The GitHub App needs to be installed to create scrapers in the normal way. If you're adding a new scraper from an existing repository on GitHub the GitHub App needs to have access to that repository. If you want to have scrapers on an organization you're part of then you'll also need to install the App on that organization.
This new way of giving access to GitHub gives you more fine-grained control. In the past you had to give "public" access to your entire collection of repositories including any organisations that you are part of. Now, you can instead choose the specific repositories that you want Morph to have access to. So in simple terms you can trust us less.
Also, by switching over to this new way it opens up the possibility of tightening the integration between Morph and GitHub. For instance, we could add a feature to automatically run scrapers whenever you push some new code to GitHub. This feature would certainly have been possible before but moving over to using a GitHub App makes implementing this more straighforward.