Comments are used when we need to write some informative text in the code that will not be treated as code.

To make a comment, write:
Tyl Symbolizer will replace the exclamation mark '!', with '≡', the comment symbol:
Say you have some mysterious maxrpm variable:
maxrpm 4000
You can describe it like:
! maximum revolutions-per-minute permitted
maxrpm 4000
When the program will run, all lines that start with a comment symbol will be ignored.

It is good practice to write comments where you want to help yourself and others understand your code.
In this guide, comments where used scarcely because almost always variable names, in full or shortened, were descriptive enouth. But in bigger or more complicated codes, or even in an error prone point at your code, make a comment that will describe what happens.