It's just sort of a keyword (called a hashtag) to make it easier to search for certain things people are talking about. If a bunch of people are talking about the TCA press tour, it's much easier to find all of those tweets if they label them somehow with "#TCA" and then people interested in that can just search for "#TCA" and get all the news. Or "#chuck" for whatever is being written about Chuck. Or, well, whatever.