Organic reach is the total number of unique people who were shown your post through unpaid distribution.

Paid reach is the total number of unique people who were shown your post as a result of ads.

Total reach is the number of unique people who saw your posts, regardless of where they saw it. If your post reaches a person organically and through an ad, that person will be included in organic reach, paid reach and total reach.

Keep in mind that the sum of post reach won't equal total reach because Pages can reach people through content other than posts. For example, if someone visits a Page after searching for it, they'll be counted in total reach but not post reach. Also, if someone sees more than one Page post, they'll be counted in post reach for each post they see, but they'll only be counted once in total reach.