what are sets?this question no are in the text book

A set is a group of similar things. Since you put this
question in the math area, I assume you are asking how sets relate to the area of
mathematics. There are many answers to that question. A set could
be:

1- A group of things that are being
counted

2 - A group of measurements, as in the set of data
values obtained while solving a problem

3 - A group of
numbers that have similar properties, such as sets consisting of all negative numbers,
positive integers between 1 and 10, numbers that can be divided evenly by
3

4 - The factors that meet given criteria, such as the
possible answers for a variable when there is more than one correct
answer