What does kill -- -0 do?

In UNIX/Linux world two dashes one after other mean end of options. For example if you want to search for string -n with grep you should use command like:

grep -- -n file

If you want to get only the line number in above case you should use

grep -l -- -n file

So the command kill -- -0 try to send signal to process with ID -0 (minus zero)

In this case it is used to denote that the -0 is not an option being passed in. If you were to do kill -0 it would think of -0 as an option not as the PID being passed in.

For example if you did ls -- -l it will look for a file named -l as opposed to just listing the directory in long form.