What it means
Reach counts people, once each, no matter how many times they saw the thing. Impressions divided by reach gives you frequency: the average number of times a person was shown it.
Why it matters to a small business
For a local business reach is the honest measure of how far you got, and it has a natural ceiling that impressions do not. There are only so many people within a fifteen minute drive of your door. Once reach stops growing, more budget is buying repeats of the same faces rather than new ones, and that is the moment to change the audience rather than the bid.
The bit people get wrong
Reach is not comparable between platforms, because each one counts a view differently, and none of them will tell you exactly how. Track it against your own previous months on the same platform and ignore any table that lines up reach from four different apps as though it were one unit.