psql command line question regarding function of single quote '

I am working my way through a book from APRESS by Matthew and Stones
entitled "Beginning Databases with PostgreSQL from Novice to Professional"

The exercises involve issuing commands through the psql command line. I am
using Windows XP on a Dell.

I notice that if I am entereing a command, but accidentally precede the
command with ' then the command is not performed, and no matter what I type,
the prompt continues to show as '> rather than -> .

I have also discovered that if I enter a second ', then the command prompt
returns to the normal -> and I may once again execute commands.

My question is, what function does the ' serve in psql command line?

Thanks!