Explain in a simple way the essence of indexes in MySQL?

Question

MySQL

I just started to study the topic of indexes, I understood how they work, but please explain in a simple way what this grouping means, what does it affect, what is its main mechanism of work?

Thank you colleagues!
PS I read the documentation, but I can't cope with the amount of new information for myself.

Answer 1

- Angela, where in our library can I find posts with type 15, status "published" dated March 13, 2015? And then readers ask, and I will go through everything in a row until the end of next week.
- No problem, Veronica, I'll tell you now (looks at the index). So, published posts with type 15 from February to September 2015 are in the third room, 10th cabinet, 5th shelf from the top. You will find a specific post yourself, there are more than 20 pieces on the shelf anyway.
- Thanks to!