[Help-bash] bash with the -x option

[Michael Convey]

In the following example, what does the -x option do?
$ p='*' bash -xc 'ls $p'
Source:
http://unix.stackexchange.com/questions/210036/filename-expansion-find-utility-pattern-matching-vs-bash-shell-pattern-matching

I can't find this option in the bash man page or anywhere online.