Impressions are calculated by the total number of potential views of the term/brand you are tracking.
Impressions calculations also take into account how many times a specific user may have seen posts containing the term you are tracking.
For example, let's say you post 5 tweets with the term #hotel and all of your #1,000 followers may see those tweets 5 times, which means your impressions would get to 5,000. In theory, that's how you'd calculate your total impressions.
A single user can deliver multiple impressions and impressions can be counted only on Social Media.
You can see the number of impressions for your mentions if you look at the Influencers menu: