The cable card is used to decode the digital cable signal. It is in lieu of a box.
You would probably want to get TWO cable cards with your TiVo (it accommodates two). That way you can be watching one channel while you record on another (or record two shows at once). (At least I assume the Premier accommodates two cards.)
ETA: Hmm... it only shows one Cable Card slot, but it says it is for a Multi-Stream card.
ETA2: It appears you only need the one card. From
Wikipedia:
Quote:
A "Multi-Stream CableCARD" (M-Card) can decode up to six channels simultaneously
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