I have noticed that, if I type \$100%\times x\$, I get $100%\times x$ and not $100\%\times x$.
Does % mean to ignore the following text or something like that?
I didn't find an explanation for the action of % in the MathJax Tutorial.
A % in this context will denote a comment. Similar to HTTP which marks comments using
<!-- a comment can go here --> or several programming languages which use slashes such as
//a comment can go here or
/*a very long comment spanning several lines can go here*/, the percent sign can be used in LaTeX and related typesetting languages such as MathJax to denote that what follows on the same line should not be rendered or executed. Comments can be helpful to help organize or explain certain code to yourself or others who come behind to read the source.
See this related question from tex.stackexchange.com.
Note that what follows on the same line is what is ignored up until an actual newline character, even in the event that you include
\\ which would otherwise be interpreted as beginning a new line.
$$a%b c d \\ e f g
h i j\\k l m$$
will render as $$a%b c d \\ e f g h i j\\k l m$$