What is ' (apostrophe) in Lisp / Scheme?

Question

I am on day 1 hour 1 of teaching myself Scheme. Needless to say, I don\'t understand anything. So I\'m reading The Little Schemer and using this thing:

Answer 1

SISC is good, but an even more lightweight online Scheme executor is http://codepad.org. It's not actually a REPL in that it's not interactive, but it's pretty close. Code you submit is executed on the server side instead of using a browser applet. And you can share code that you're running by short URL.

The about page on codepad says it uses "MzScheme v372 [cgc]".

I use codepad for all kinds of quick snippet testing (including testing code samples for SO answers!).

For the quote syntax, the difference can be seen using code like this:

(let ((x 5))
  (display x) (newline)
  (display 'x) (newline))

This displays:

5
x

In the first case, x is evaluated and passed to display, which prints 5. In the second case, the symbol x (which isn't the same thing as a character string) is passed to display, which prints the name of the symbol.