The six horn blasts that were blown before Shabbos.
Announcing that Shabbos is coming some time before Shabbos: At the time that the Jews were properly settled the Sages instituted to blow six blasts from a high area, in order to stop the people from work [before Shabbos]. However now it is accustomed in the holy communities, that when it gets close to Shabbos, [which is about] a half an hour or an hour [to Shabbos], a community messenger announces to [the public] for them to get ready for Shabbos.
[The reason this is done so early before Shabbos, as opposed to near sunset is] because one is required to add from the weekday to Shabbos, as will be explained in chapter 261. It is proper to follow this custom in all places (that delay the night prayer).
Stopping the people from work before Shabbos: In any event [whether or not ones community follows the above custom] a person must be sent in order to stop the working people from doing work [of physical labor], beginning from Minchah and onwards, which is two and half hours before Shabbos, as explained in chapter 251.
Stopping business before Shabbos: Also the businesses, even though they are permitted to manage it after Minchah, as explained there, nevertheless it is proper to send someone to close the shops about an hour before Shabbos, as many times [if this is not done] a grave dilemma derives from this, as at times an important person comes into the shop, and the business negotiations continue until actual night, and the collar lies on the heads of the community, as the Sages have expounded from what it says in the verse, “And I will put it on their necks”.
Stopping laymen from before Shabbos: As well the majority of the community needs to be warned [of the incoming Shabbos], as they give themselves permission that as long as they have not said Barechu, they do all that is needed for Shabbos even though it is already night [and Shabbos has begun. For this reason the saying of Barechu should be preceded [as much as possible to be said before Shabbos], and one should not delay saying it due to an important person which has still not come to the Shul, as it is [a greater] merit for him that Shabbos not be desecrated because of him, and a wise man foresees these problems [and thus will not delay coming to Shul].
 There is a Mitzvah to accept Shabbos earlier than the actual time which it begins. Thus the announcement is made early so people can be prepared for Shabbos with enough time to accept Shabbos early.
 The explanation of this is seemingly as follows: Back in the day it was very common for the community to pray Maariv before Shabbos, and by doing so they would in essence accept Shabbos early. By these communities there is not such a need to have an announcement made early, being that they anyways have a set custom to pray Maariv early, and will thus in any event fulfill the mitzvah to accept Shabbos early. However those communities which pray Maariv after Shabbos has begun should follow the above custom in order so they be reminded to accept Shabbos early.
 This refers to stores, or any other job which involves buying and selling. However earlier only work of physical labor was discussed.
 Lit. stumbling block. Meaning it causes one to transgress a prohibition.
 Lit. an aristocrat
 Meaning the responsibility lies on the leaders of the community.
 From this verse the Sages learned that the transgressions of the community which can be stopped by the community leaders, give liability to those leaders which did not try to stop them.
 Barchu is the introduction to Maariv, and when prayed before Shekiah, represents the acceptance of Shabbos. However once sunset has arrived Shabbos automatically begins and is no longer dependent on the saying of Barechu by Maariv.
 Lit. “a wise person has eyes in his head”.