Organic Reach
Organic reach is the number of people who have seen your post through unpaid distribution. Keep in mind that after you have boosted an existing post, the number that represents organic reach might go down. This is because the people who saw your Page’s post before it was boosted and then saw your post again after it was boosted are now counted toward paid reach.
Paid Reach
Paid reach is the number of people who have seen your post because of your boost.