the use of $$string$$

Hi,
I just discovered that I can use $$string$$ to account for the problem of
single quotes in the string (or other strange char's). However, I noticed
that the table field contained E'string'. I actually tried to find info on
this but I did not find anything.

Could someone explain what it means or better provide a web link for me to
discover the info.

Thanks in advance,
Johnf