Its like a dinner plate but with dnb on it.
Actually:
Its a record you can play on a deck except prepared in a different technique, its acetate not vinyl.
Its used for short, trial or exclusive runs. So producer A will give Dj B a dubplate of a new track months before it ever gets circulated more widely. If ever.
Bootlegs get dubbed, vip tracks get dubbed, etc. Some tracks will never get released and will only appear as dubplates. Dubplates seem to be getting replaced by cds with tracks transferred via AIM.
Dubplates don't last as long as vinyl... Sometimes dubplate can refer to cds with exclusive, VIP or prerelease tracks.