Plate in this case is wrought gold or silver, usually in the form of ornamental versions of tableware - things like literal plates, vases, patens, chalices, candlesticks etc. Many of these would have originated from the Church and been taken into the king's treasury during Henry VIII's dissolution of the monasteries.

Project Gutenberg has a good text from Cornell University giving descriptions of various items of English plate, if you want to get a picture of the sort of thing in question.