The person can never see your posts or profile
As soon as you block, someone this person can never visit your profile or see your posts. This person will not see anything related to you again.
You will never see that person’s posts
As soon as you block someone on Facebook, you will stop seeing their posts and you will also be unable to visit their profile.
Profiles become un-clickable
If you had a message in your inbox from that person, the person’s name will become un-clickable and his name will change to ‘Facebook user’. The person will see the same thing as well in his inbox.
You won’t be able to tag each other
When you block someone he won’t be able to tag you in a post as your name will not show up. The same will happen to you too as you won’t be able to tag this person in a post.
You won’t be able to invite each other to events
The person you blocked won’t be able to invite you to events or groups. The same will be true for you as well.
You will not become friends
If you block a person then unblock them, you will still have to add each other as friends once again. The Facebook block deletes the friendship as well. (See What happens when you block someone on Whatsapp?)
You won’t be able to message each other
When you block someone on Facebook, the person won’t be able to send you any messages. If you had a previously opened conversation in your inbox, there will be no option to send messages. The same happens to you as well.
The person will see an un-clickable tag
If someone tags you in a post and a blocked person sees that post, he will only see your name dimmed and won’t be able to follow the link back to your profile.
You will disappear
According to that person you blocked, you would seem like you disappeared from Facebook completely. The same will happen to you as you will never come across that person as well.
They won’t get notified
The person won’t get notified that you blocked them but they might realize one day it happened when they fail to reach you if they ever tried or if they saw your name tagged in a post and found it un-clickable. (See What happens when you block someone on Instagram?)