Click on “Analytics”, the app automatically calculates the number of Reach and Impressions. Your reach is the sum of followers of the users who have participated in the conversation. For instance, if 10 people retweet your post and they have 1,000 followers each, your reach will be 10,000.
The impressions are calculated based on the number of followers for each user who tweeted using your hashtag or keyword. If the tweet is a reply, then the impression would be counted as 1, because replies are made to a single user.