How to understand the wording in a book on algorithms?

Question

Programming

Hello! I am studying the book by D. Knuth "The Art of Programming", there is such a formulation there, which, no matter how I try to understand it, does not lend itself to my mind.
What are sets and subsets? Why were these symbols chosen, how to learn to understand them? Obviously, there is an obvious gap in knowledge in this place, and this book is too specialized. I would like to learn to understand all this: not to shine with knowledge, solving scientific problems, but at least to understand what is at stake.
I ask those who know to share a book or a course on YouTube that would help me understand all this and improve my knowledge, fill in the missing gaps, so that I could at least read this book with a smarter face.
Thank you!

Answer 1

This is mathematics, you see how gaps in knowledge make themselves felt, read on Google about sets and subsets.
And the designations can be any, here they are apparently standard

Answer 2

1. Parse all incomprehensible words
2. Reread until awareness comes

Answer 3

And what is it for you ... to understand Knut, moreover, translated, how to listen to Caruso in Rabinovich's rehash :). In general, everything, and even a surplus, that can be useful in the math for a programmer - this is the book. If you want to go into science, you yourself will understand what to read / learn.
In general, Knut is like "War and Peace", few people read it in full, but everyone advises :)