The syntax of ANSI-C quoting is something like
$'\0x009' or $'\t'.
This is useful when you want to input a tab on the command line as an input parameter (tab key is mapped to auto-completion in bash).
psql -A -F $'\t' ...
will set the field separator to tab.
This is documented in http://www.faqs.org/docs/bashman/bashref_12.html as:
"Words of the form $'string' are treated specially. The word expands to string, with backslash-escaped characters replaced as specified by the ANSI C standard. Backslash escape sequences."
For tab, you can also input a CTRL-I (type CTRL-V-I).