1 You stop seeing their posts
The first use of unfollowing someone is that their posts would not appear on your newsfeed. You would not be able to see any of their updates until you start following them again. If you open their profile, you might be able to see some of their posts depending on their privacy settings.
2 They will remain on your friend list
Unfollowing is very different from blocking a person from your Facebook. When you unfollow someone, they would still be on your friend's list, nothing will change, and they would not even get to know that you unfollowed them.
3 They don't get a notification
If you unfollow a friend, group, page, or non-friend them, they do not get any information about the same. In fact, if you decide to follow them back, then also they do not get any notification regarding that. In this complete process, the other person remains unaware.
4 They see your posts
Unfollowing someone is just a one-way journey; in this, the other person would be able to see your posts and stories. The person that you unfollow would be able to see all your posts and stories normally as before. Your posts and feeds will appear in their newsfeed if you are friends with them. (See what happens when you unmatch someone on Tinder?)
5 They might guess it
In case if you frequently post on Facebook a bit never interact with posts of a particular person, then they might guess that you have unfollowed them.
6 You will get birthday notifications
Unfollowing only affects the posts in the timeline. You would get their birthday notifications as before there would be no difference in that. (See What happens when you unfollow someone on Instagram)
7 They can message you normally
Messaging would remain the same as both of you would be able to send messages to each other. The person you unfollowed would be able to interact with you in a similar manner before you unfollowed them. Other actions, such as posting comments, likes, and tagging everything would remain unchanged.
8 Everything remains the same
All the things would continue as before you unfollowed that person, and there would be no change in the way Facebook works.
9 Who all can be unfollowed
You can unfollow friends, non-friends, pages, and groups on Facebook.
10 Like and comments
The best part is that nothing would change for likes and comments. You can like and comment on their posts and vice versa.