Shabbat, Chapter One, Mishnah Two
After an introductory mishnah, the Mishnah continues with laws relevant to the day before Shabbat. Our mishnah deals with things that one shouldnt do once the time for the afternoon prayer, minhah, has arrived. The problem is that if one becomes engaged with one of these activities, he may become swept away and forget to pray minhah.
These laws apply not only to the day before Shabbat, but to any day of the week.
The minhah referred to here is from the time of 9 1/2 hours, the day being divided into 12 daylight hours. So on a day where daylight begins at 6 AM and ends at 6 PM, minhah is from 3:30 in the afternoon. In the Northern Hemisphere the hours will be longer in the summer months and shorter during the winter. We should note that one can also pray minhah at an earlier time of day, from 6 1/2 hours.
1) One may not sit down before a barber near Minhah until he has prayed.
2) One may not enter the baths or a tannery, or [sit down] to eat or [begin] a court case.
3) But if they began, they need not break off.
4) One must break off for the reading of the Shema, but not for prayer.
Section one: One shouldnt begin to get a hair cut right before Minhah, lest the hair cut take a long time and he forget that he must pray his minhah prayers. After the sun has set, he will not be able to pray minhah.
Section two: Similarly, a person shouldnt begin any of these activities close to minhah time, lest he forget to pray. A tannery is an example of any type of work which is somewhat complicated and may cause him to forget that the time to pray has arrived.
Section three: However, if he began one of these things, then he need not stop in order to pray. Rather he may complete his haircut, etc. and then pray afterwards.
Section four: This section explains that one must stop what he is doing to recite the Shema but one doesnt have to stop for prayer, otherwise known as the Shmoneh Esreh, or the Amidah. The reason is that the obligation to say Shema is from the Torah whereas the obligation to recite the Shmoneh Esreh is only derabbanan, of rabbinic origin.