Not sure if such term exists in English but it does in Spanish: it is the most extreme case of emergency and, as it name suggests, it implies that the town (or in this case the whole country) is under siege and hence all normal liberties are severely limited.
The decree implies that for the next 45 days, freedom of speech and movement will be extremely restricted, public gatherings are totally forbidden as well. The decree also commands the forcible evacuation of any public building occupied by demonstrators, closure of media "that offend human dignity, public servants or breach the law" and the arbitrary arrest of "suspect" people.
The decree was approved on September 22nd, just after the legitimate President Manuel Zelaya returned from his forced exile, but has only been published today and publicly broadcast in all TV and radio networks.
Zelaya in turn has protested the measure as "a barbarity" and has appealed to Parliament to suspend it and reach out to his offer of dialogue. He has also made an appeal for a pacific demonstration lasting for the next 24 hours.