Reach represents the total number of people who might see posts containing your monitored keywords. You can think of reach as the number of unique people who can see pieces of content that contains your brand name, company, product, etc.
For example, if you post something on Twitter and your Twitter account has 2500 followers, the reach for that post will be 2500.
If you're interested on how impressions are calculated, you can find more info about it here.
Within BrandMentions, you can find reach data in both Dashboard and Influencers sections.
On the dashboard you can see the total reach for your mentions, as well as a chart that gives reach data per day.
Within the Influencers' section you can see reach data for each of your influencers.