What is a set? Well, simply put, it's a collection.

First we specify a common property among "things" (we define this word later) and then we gather up all the "things" that have this common property.

https://www.mathsisfun.com/sets/sets-introduction.html

what does property mean in this context?

I don't get the proper meaning of characteristics and properties in science and maths.

Sense 2 at https://www.lexico.com/en/definition/property

*An attribute, quality, or characteristic of something.*

The properties of "1" are (for example)

- an integer
- an odd number
- a positive number
- a number which multiplied by another (second) number, results in a number equal to the second number.

The properties of "2" are not the same as "1". However they share two properties: #1 and #3.