Impressions is a raw number of times your ad has been display. In the case of Google, they provide you viewable impressions, meaning the user must have scrolled down to the ad if it appears below the fold.
Reach and frequency numbers are more meaningful numbers for branding. Reach tells you how many unique people your ad has been shown to while frequency tells you the average number of times each person has seen your ad.
So an example of campaign goals would be a reach of 100,000 users and frequency of 20x. That would end up giving you 2,000,000 impresssions.