Reggae musicians in Jamaica record one-off tracks in small studios which they then try out at clubs, or parties - they are called 'plates'. Dub reggae is music mixed differently - with the bass and drums brought right forward, and various echoes and effects put on - this can be done as part of a 12" mix with a standard song, and a 'dub' on the end, or the whole 'dub' on the 'B' side. So a 'dub plate' is simply a one-off single with a dub mix on it.
Oh, and there are no 'stupid' questions - there are ones you know answers to, and ones you don't - this time you didn't know, I did - next time it could be the other way round, so don't worry about it!
Used in the manufacturing of records. It is an acetate cut directly by the cutting stylus onto a plastic coated metal disc. It is the first 'record' in the manufacturing process and is used to assess the quality of mixing and mastering before pressing.