Reach is the number of distinct, unique people who saw a piece of content at least once, in contrast to impressions, which count every individual time the content was shown including repeat views.
Reach tells you the size of the audience you actually touched. For a small business deciding whether a channel is worth the effort, reach is a clearer measure of audience than impressions, which can be inflated by the same people seeing something many times.
Reach and frequency trade off. A fixed budget can reach many people once or fewer people many times. Awareness usually wants broad reach; retargeting usually wants concentrated frequency.
Reach without engagement is a warning. Touching a large audience that never reacts often means the content or the targeting is off, so reach is a starting point, not a result.
A florist's social posts in a month generate 12,000 impressions but a reach of 4,000. On average, each person who saw the content saw it three times.
That tells the florist its content is reaching a modest local audience repeatedly, which is fine for staying top of mind but limited for finding new customers.
If the goal were awareness, the florist would want to widen reach. If the goal were nudging existing followers to book, the current frequency is doing its job.
In-House plans social and paid reach against the goal through the social and advertising agents, choosing broad reach or concentrated frequency depending on what the campaign is for.