The idea of "sets" may come from "differentiation" (การแยกความแตกต่าง). We need to know "what things are the same" (สิ่งไหน เหมือนกัน) for our 'purpose', and what things are not the same as others.

What do we do when we separate 'mangoes' and 'guavas' into 2 piles (or sets: a set of mangoes, and a set of guavas)?

How do we tell which fruit go to which pile (set)? How do differentiate fruits? By 'shape', size, colour, smell, taste, ...?

In mathematics, we say we describe the fruits by their 'properties'. We say fruits that have the same 'list' of properties are the same fruit (or in the same set). And fruits that do not have properties in the list (of properties) or have some properties not on the list (of properties) are different fruits -- not in the same set.

From here we can talk about, fish instead of fruits. We can go on to cars, letters of the alphabet, numbers, 'things' and ideas, ... as abstraction of anything. Thus we come to learn 'mathematics' as 'an art of abstraction'.