Paragraph 1 of Section 251 established as a principle that one should not engage in gainful employment on Friday afternoons. The obvious purpose of this principle is to ensure that everyone leaves enough time to make all the necessary preparations for Shabbat. Paragraph 2, now, will 'annotate' that principle.
Rabbi Karo states that despite the principle enunciated in paragraph 1 it is permitted to perform last-minute emergency tasks, and these may be done 'all day' - i.e. up to the very last minute before the onset of Shabbat. He provides as an example the mending of clothes. Part of the 'Shabbat delight' [in Hebrew: oneg shabbat - making Shabbat a delightful day - is wearing fine clothes. Wearing weekday clothes on Shabbat would be unthinkable. Therefore, if on Friday afternoon a tear is discovered in one's Shabbat best it is permitted to mend the tear. Rabbi Karo then immediately generalizes his specific example: any article that will be used on Shabbat that needs mending or similar attention may be repaired on Friday afternoon. Obviously, two things should ideally be borne in mind: that the need for the repair work is sudden and could not have been done earlier in the week, and that the repairs are needed for Shabbat. However, the second consideration overrides the first: if the repair is needed for Shabbat it may be done on Friday afternoon even in it could have been done earlier in the week.
One might think that this permission is given only for oneself, to make last-minute repairs. But Rabbi Moshe Isserles adds a note (the first of two in this paragraph). Not only may one do necessary repairs for one's own Sabbath needs on Friday afternoons, but one may also perform a similar service for others - provided that no payment is received. If the repair is really needed for Shabbat, even if only to make it 'a delightful day', the work may be done for others on Friday afternoons.
Later authorities extend this permission even further. When the person doing the work is poverty-stricken and needs the money in order to provide what he or she needs for Shabbat they may even receive payment for services rendered. Most authorities are even more generous: not only may payment be accepted for emergency repairs on Friday afternoon when the money will provide for Shabbat necessities; it may also be accepted by the poor person when the money will go to providing 'extras' for himself and his family that will add to the 'Shabbat delight'. The only proviso is that the money be used to provide things for Shabbat which would otherwise not be available.
Clearly there is a danger that people might take advantage of this leniency. They might convince themselves that they really need the money for Shabbat or that the repair might be considered necessary for Shabbat and so forth. This must be left to the individual's conscience. Presumably the preamble to paragraph 1 of this section may serve as a warning to such people: