Reach is the number of unique users that have seen your ad whereas impressions are how many times your advertisement was shown on screen. If an ad reaches 25,000 impressions and a reach of 5,000, that means that 5,000 unique users saw the ad an average of 5 times. The average amount of times a unique user sees your ad, 5 times in this example, is referred to as frequency. Frequency is an average because it is not guaranteed that someone saw the ad every time it appeared on screen: if the ad appeared on the bottom of the page but the user never scrolled down so that the ad was in their view.
Reach, impression and frequency in the form of an equation would look like this:
25,000 impressions / 5,000 unique users = an average frequency of 5
When the goal of a campaign is to show your advertisement to as many users as possible, its campaign objective will be a reach campaign.
When the goal of a campaign is to create retention around a product or brand, its campaign objective will be a brand awareness campaign. Retention will be built around someone seeing an ad for the same company/product multiple times, meaning the ad will want to have a high average frequency. This in turn will help make that company or product recognizable.