If your gift card was given for a specific restaurant, then no, it does not need to be activated.
If you've been given the option to choose your own restaurant, you will need to activate your gift card before use. Visit https://gifts.opentable.com/activate, and follow the onscreen instructions to activate your gift card.
Do I need to print out my gift card in order to use it?
Nope! Simply present the emailed certificate to the server on your smart phone or other portable electronic device, or provide the server with your unique gift certificate number. The server will be able to apply the amount to your bill using the gift certificate number.
Can I change restaurants once I've activated my gift card?
Unfortunately not. Once a gift card has been activated for a specific restaurant it must be used at that restaurant.