What is traffic reach? How do I calculate it and what is it telling me?

I will respond to the question from an advertising point of view.

In advertising, reach is the total number of people exposed to the advertising medium/advert at a specified time. Reach has often been confused with the actual number of individuals who consume the advert or promotional communication. The term traffic reach is mostly synonymous with paid online advertising. Online advertising engines such as Facebook ads, Google Adwords, and Twitter ads have traffic reach as one of the metrics determining the cost of the advert. Traffic reach in online paid advertising is mostly an estimate of the number of people who will be accessing the platform at the time when the advert will be running. For instance, Facebook will determine the number of individuals exposed to the advert at the time when it will be live and running. The estimate will also be based on the target selected by the advertiser using available demographics such as gender and geographical location.

Online advertising engines use an algorithm that forecasts the number of people based on the advertiser's targeting and the system's historical data. However, an exact figure can be determined by counting all individuals exposed to the advert. Thus, traffic reach provides information that is essential in determining the effectiveness of the advert with regard to how many people are likely to consume the advert.