dh_installmenu is a debhelper program that is responsible for installing files used by the debian menu package into package build directories.
It also automatically generates the postinst and postrm commands needed to interface with the debian menu package. See dh_installdeb(1) for an explanation of how this works.
If a file named debian/package.menu exists, then it is installed into usr/lib/menu/package in the package build directory. This is a debian menu file. See menufile(5L) for its format.
debhelper(1) update-menus(1) menufile(5L)