What is the difference between the metrics 'People reached' and 'Ad impressions'?

Ad impressions – The total amount of times the ad was viewed on social media.

People reached – The total amount of people the ad was viewed by.

For instance, if the ad has been viewed 4 times by 3 people, the total amount of 'People reached' is 3 while the 'Ad impressions' amount to 4.

If one and the same person views the ad several times, the amount of 'People reached' will not increase but the total amount of 'Ad impressions' will.