[gentoo-user] overlay eclass overrides eclass from PORTDIR: what is it?

[b.n.]

Hi,
Syncing today gave me this message:

 * Overlay eclass overrides eclass from PORTDIR:
 *
 *   '/usr/portage/local/layman/desktop-effects/eclass/bzr.eclass'
 *
 * It is best to avoid overridding eclasses from PORTDIR because it will
 * trigger invalidation of cached ebuild metadata that is distributed with
 * the portage tree. If you must override eclasses from PORTDIR then you
 * are advised to run `emerge --regen` after each time that you run `emerge
 * --sync`. Set PORTAGE_ECLASS_WARNING_ENABLE="0" in /etc/make.conf if you
 * would like to disable this warning.

Could anyone explain me what does it mean in practice?

Thanks,
m.