Favoriting just likes the feed, like you can like a post. Pinning makes the post more easily accessible to you. Importing? If you mean subscribing, then it just means the feed is one you're following.

Thanks. I hadn’t noticed that the app menu changes depending on whether I am already subbed to a feed.

The difference between pinning and following is pretty unclear. That aspect of the UX seems pretty dynamic and likely to change within the next few weeks/months.