Reach counts distinct people, not views. If one person sees your post three times, that is one unit of reach but three impressions. Platforms usually split reach into followers and non-followers, which tells you how much of your distribution came from your existing audience versus discovery surfaces like the explore feed or the for-you page.
Reach matters because it is the ceiling on everything else: no one can engage with, click, or remember a post they never saw. Growing reach among the right people, rather than just any people, is what turns content into a marketing channel instead of a vanity exercise.
Non-follower reach is the growth lever. Content that earns shares and saves, uses discoverable formats, and rides relevant topics tends to reach beyond your current audience. Track the follower vs non-follower split over time to see whether you are compounding or just talking to the same room.