What is a Twitter impression?
Impressions on Twitter is a total tally of all the times the Tweet has been seen. This includes not only the times it appears in a one of your followers’ timeline but also the times it has appeared in search or as a result of someone liking the Tweet.
Simply, Twitter impressions show how many total times people have seen your tweet .
In other words, 500 impressions = your tweet has been seen 500 times.
It’s only when you look at impressions with other metrics like reach or engagement that you can understand more on where you can improve.