Those are sequences of characters sent by your terminal when you press a given key. Nothing to do with bash or readline per se, but you'll want to know what sequence of characters a given key or key combination sends if you want to configure
readline to do something upon a given key press.
When you press the A key, generally terminals send the
a (0x61) character. If you press
<Tab>, then generally send the
^I character also known as
\t (0x9). Most of the function and navigation keys generally send a sequence of characters that starts with the
^[ (control-[), also known as
\e (0x1b, 033 octal), but the exact sequence varies from terminal to terminal.
The best way to find out what a key or key combination sends for your terminal, is run
sed -n l and to type it followed by Enter on the keyboard. Then you'll see something like:
$ sed -n l
The first line is caused by the local terminal
echo done by the terminal device (it may not be reliable as terminal device settings would affect it).
The second line is output by
$ is not to be included, it's only to show you where the end of the line is.
Above that means that Ctrl-Up (which I've pressed) send the 6 characters
A (0x1b 0x5b 0x31 0x3b 0x35 0x41)
terminfo database records a number of sequences for a number of common keys for a number of terminals (based on
TERM=rxvt tput kdch1 | sed -n l
Would tell you what escape sequence is send by
rxvt upon pressing the Delete key.
You can look up what key corresponds to a given sequence with your current terminal with
infocmp (here assuming
$ infocmp -L1 | grep -F '=\E[Z'
Key combinations like Ctrl-Up don't have corresponding entries in the
terminfo database, so to find out what they send, either read the source or documentation for the corresponding terminal or try it out with the
sed -n l method descrive above.