It finds subdirectories of usr/local in the package build directory, and removes them, replacing them with maintainer script snippets (unless -n is used) to create the directories at install time, and remove them when the package is removed, in a manner compliant with Debian policy. These snippets are inserted into the maintainer scripts by dh_installdeb. See dh_installdeb(1) for an explanation of debhelper maintainer script snippets.
If the directories found in the build tree have unusual owners, groups, or permissions, then those values will be preserved in the directories made by the postinst script. However, as a special exception, if a directory is owned by root.root, it will be treated as if it is owned by root.staff and is mode 2775. This is useful, since that is the group and mode policy recommends for directories in /usr/local.