Reach tells you how many unique individuals saw your post or content. If 200 people saw your Facebook post, your reach is 200. Don't confuse reach with impressions — impressions count total views (one person can generate multiple impressions), while reach counts unique people.
There are two types: organic reach (people who saw your content for free) and paid reach (people who saw it through advertising). For new businesses, growing reach should be a top priority — the more people who see your brand, the more potential customers you create. Facebook's organic reach has dropped to around 5% of followers on average, which is why many businesses invest in paid reach to expand their audience.