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.