This is useful when you want to read a file that’s in the same directory as the current script, for instance.
$BASH_SOURCE holds the current script, including a path. That could be a relative path or a full path.
% says “strip off a glob from the end of the value, the smallest glob that matches”
/* is a glob, which will match the
/ at the end of the file’s directory plus the filename.
So this expression gives you the directory of the bash script you’re currently in.
This trick brought to you by Avdi.