This Mishnah speaks about things that one should not do on Friday afternoon, close to sunset - but commentators note that this applies also to weekdays, for a very similar reason.
Simply put and condensed, one should not enter into any (possibly lengthy) time-consuming activity close to Shabbos. The examples given include visiting a barber, a bathhouse, one's place of business (maybe you'll find a possible impending loss of profit and try to fix it), sitting down for a full meal or sitting down to judge a courtcase.
Because if you do begin these things - even if there's theoretically enough time for you to finish - things might go awry and when Shabbos comes in you'll find yourself still embroiled in the middle of it.
However, if you began one of these (or similar) without realizing how little time you had left, you don't have to stop in the middle. (No needing to walk around for a day with only half your hair cut.)
The Mishnah, having mentioned interruption, then digresses-
(Side-note. Digression is something both the Mishnah and the Talmud do. Often extensively.)
-to say that if two Torah-scholars are learning, they must stop in the morning to say the Shema prayer. But they do not have to stop their learning to say the Amidah (the silent standing prayer).
Posted by adam at May 9, 2006 09:55 AM