Apple - What is the meaning of the "§" symbol (on the upper left corner of the mac keyboard)?

"§" is the section sign, as answered by @Lizzan.

Excerpt from Wikipedia:

The section sign (§) is a typographical character used mainly to refer to a particular section of a document, such as a legal code. It is also called "double S" and "sectional symbol".
It is also sometimes used to link to a footnote where the asterisk (*) is already in use on a given page.

It's a Section sign, and is used in Minecraft for color formatting codes.

As noted, it is the “section sign” symbol. I had no idea it was on some keyboards! On my American Apple keyboard, I can type it as ⌥6 (Option-6).