A timbre, originally, was a dry stamp, as was a sello that litterally means seal. By extension a timbre is a stamp/seal issued by the state and/or for proof of taxation. A stamp in the sense of a postage stamp - as the English used to call it adhesive postage stamp - is quite rare in modern Spanish. That, as naufrago writes, is "sello."
(as vayolene just posted.)
The RAE lists even more definitions. One of those is the English "timbre" that a bell also has.