Reach is the total number of unique Twitter users that tweets about the search term were delivered to. Exposure is the total number of times tweets about the search term were delivered to Twitter streams, or the number of overall impressions generated.
Currently, there is no way to know whether or not a particular user has actually read a particular Tweet. When we say "impression", we mean that a Tweet has been delivered to the Twitter stream of a particular account. Not everyone who receives a tweet will read it. This is similar to impressions as calculated for many forms of paid advertising; even though an ad was displayed on a website, there is no guarantee that a person actually saw it. Both reach and impressions should be treated as directional metrics to give you an idea of the overall exposure the tracked term received. You should use these metrics to get a sense of the size of your potential audience, and use engagement metrics like retweets, clicks and replies to gain a more complete understanding of your impact.
There's more detail here about how we calculate those numbers.