An "Official Poll Card" is a postcard sent to those people on the Register of Electors before a UK election. The card tells you the date of the election and where you can vote. The date of the election is called the "Polling Day", and the place where you vote is called the "polling station". The polling station is open for you to cast your vote during the "Polling Hours" given on the card.
I like to think of my Poll Card as an invitation to a very special event celebrating democracy. It's not RSVP; come as you are. As a courtesy to your hosts, please take your invitation with you. But, as it says on the invitation, you do not need your Poll Card when you go to vote.