"Twitter is a free social networking and micro-blogging service that enables its users to send and read messages known as tweets.
Tweets are text-based posts of up to 140 characters displayed on the author's profile page and delivered to the author's subscribers who are known as followers. Senders can restrict delivery to those in their circle of friends or, by default, allow open access. Users can send and receive tweets via the Twitter website, Short Message Service (SMS) or external applications." [source: Wikipedia]
It is basically a way to publicly broadcast text messages. Consider it a text message that people can read publicly, or a post that is forwarded to your mobile phone.
BApk organizers, moderators and coordinators are going to post updates about on- and off-line activity, and, if you register your device, you are going to be updated in real time.