In Union Metrics Twitter reporting, we define reach as the total number of estimated 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 potential impressions generated.
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, so you should consider this a measure of potential impressions. Both reach and impressions should be treated as directional metrics to give you an idea of the overall exposure the tracked term received. 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. And if you're interested in how our potential impressions metric compares to the impressions number calculated by Twitter in your account analytics, read this.