it's `cd $_` with the underscore, it repeat the last argument.
from bash man page
> _ At shell startup, set to the pathname used to invoke the shell or shell script being executed as passed in the environment or argument list. Subse‐
> quently, expands to the last argument to the previous simple command executed in the foreground, after expansion. Also set to the full pathname used
> to invoke each command executed and placed in the environment exported to that command. When checking mail, this parameter holds the name of the
from bash man page
> _ At shell startup, set to the pathname used to invoke the shell or shell script being executed as passed in the environment or argument list. Subse‐
> quently, expands to the last argument to the previous simple command executed in the foreground, after expansion. Also set to the full pathname used
> to invoke each command executed and placed in the environment exported to that command. When checking mail, this parameter holds the name of the
> mail file currently being checked.