FeedItem: It represents the entry in the feed, such as includes text posts, link posts, content posts, and changes in a record. It is a type of feed element.
Salesforce Feed is an interface wherein you and other Chatter users in your org can view profiles, updates, status messages, topic detail pages, on record detail pages, objects, and groups.
In General if you want to understand Feed, feed is like a post (like a fb post) which you can share, like, comment and the object which store this is feed item.