In social media, reach refers to the size of the potential audience for a message, or the maximum number of people who could have been exposed to a message. It's a lot like how newspapers and magazines use reach to measure circulation numbers or how TV programs use Nielsen ratings. Reach counts the size of your unique potential audience.
Reach is essential to campaigns, because it will help you understand the potential impact of your tweets. It is important for quantifying the size of your message's universe and understanding if a campaign is successful. You can think of reach as the denominator in your measurement equations. Use reach alongside engagement numbers like clicks, retweets, or replies to calculate an engagement percentage (i.e. how many people participated out of the possible audience of your campaign).
To calculate reach, we use a robust statistical model built on years of analyzing hundreds of millions of Twitter posts. To learn how we calculate and define reach and exposure (impressions), read this. If you're interested in the history behind our reach calculation (dating back to our TweetReach days!), there's more on our blog.