What it means
Organic means unpaid: your posts, your search listings, people tagging you, word of mouth. Organic reach is how many people that gets in front of, and it is decided by the platform's ranking rather than by how many followers you have.
Why it matters to a small business
It is free in money and expensive in time, and small businesses regularly get that trade the wrong way round. The important thing is that none of it is yours. Followers live on somebody else's platform under rules that change without notice, which is why the one asset worth building alongside it is a list of email addresses or phone numbers you actually hold.
The bit people get wrong
Follower count is not reach and has not been for years. An account with 4,000 followers can reach a couple of hundred of them on a given post. Judge a post by how many people it reached, not by how many people could theoretically have seen it.