Impressions are the number of times your Facebook post/ ad has been displayed to your target audience.
Think of your Facebook page as a store. If the same person walks by your store 5 times a day while walking their dog, it is counted as 5 Impressions. The Reach, in this case, is only 1. After walking by your store several times, the person will start noticing your store name, what you sell, etc and will eventually step inside to do business with you.
Though Impression is not a good representation of the number of people who see your post, high Impressions could indicate growing brand awareness among your target audience.
If you want to know the number of unique people who have seen your post, refer to the Reach metric. You can use the Impressions and Reach metric together to determine the Frequency of ads/posts which we will touch on in our next Word of the Week!