the debate is about the political system in hong kong going forward and whether beijing or the public holds control in the future.
beijing has promised universal suffrage in 2020 (and 2017 for the election of the chief executive), but of course it remains to be seen what that means in beijing's eyes. beijing also promised a gradual and orderly process towards it, but it's unclear what that means and at what speed.
the current debate is about the arrangements for the 2012 elections which are a first step in that direction. the government made a proposal for which they need 40 people out of 60 to get it passed (they only have 36 or so as the democrats hold the remainder). the democrats have already turned down a (poor) proposal in 2005 which then stalled the discussion for 4-5 years until now.
"act now" is the governments way of telling the democrats to move on and accept the current proposal. in the last few days there seems to be agreement on a compromise proposal (actually surprisingly accepted by beijing), which could mean that more than 40 legislators will vote for it. the remaining democrats who will continue to oppose it are the ones you will see roaming around central in the next few days.
that's my take on things...