It's possible that for some of you, ENIs are not clear just yet.
That's okay, they are an advanced concept in AWS and one that can take time to master.
If you'd like to learn more on ENI, I found that many students were helped by reading this blog: https://aws.amazon.com/blogs/aws/new-elastic-network-interfaces-in-the-virtual-private-cloud/
I hope this will help you. If you still don't understand it, no worries, this is not a blocker for the course. Keep on going, and come back to this concept towards the end.
Happy learning!