The interactive map shows individual Tweets as a series of horizontal bars, which are sized by the level of engagement that each Tweet received. Selecting a Tweet, or searching for a specific term, reveals the corresponding state-wide engagement level by the color of the state. Darker colors correspond to higher rates of engagement, which is measured by the number of favorites and retweets.
Alternatively, one can directly select a state, after which the map then shows the particular issues that resonate most in that state.
More detailed information about this Twitter visualization is available here.