Put simply it's what allows EasyGift to work on your store. Without activating the app embed block EasyGift cannot run on your store.
The first step in the installation process of the EasyGift app is to activate the App Embed into your theme. Follow these steps to activate the EasyGift App Embed:
There's an extra step for users who are using the unsupported version 2 of EasyGift. You'll need to select Version 2 from the options under the EasyGift App Embed setting.