PostEaseThe direct answer
Organic reach is the number of people who see a post through unpaid distribution: the platform’s own recommendation or feed system deciding to show it, with no money spent to boost it. It is usually contrasted with paid reach, which is reach bought through advertising.
Why organic reach varies so much between posts
Two posts from the same account, posted the same week, can have very different organic reach. The system is estimating how likely each post is to hold attention, not distributing views evenly across everything an account posts. A strong post can outreach an account's own follower count; a weak one can undershoot it.
It is a starting point, not a growing baseline
Organic reach for a given account is not a fixed number that grows alongside a follower count. A larger following does not guarantee proportionally larger reach on every post. Reach depends more on how a specific post performs than on how many people have followed the account historically.