Dedekind section. What is the meaning of it?

Question

Mathematics

I started to solve Demidovich, and there this is. Absolutely no idea what it means.
+ If I skip this topic, will there be any problems in the further study of mathematical analysis?

Answer 1

One of the ways to get real numbers from rational numbers.
There will be no problems.
However, these suprema and infima would be worth tormenting.