You gain access to the Citi ThankYou Rewards program by being a card member of certain Citibank credit cards such as the Citi Strata Premier Card, the Citi ThankYou Preferred Card and the Citi ThankYou Prestige Card. In the program, you can EARN Citi ThankYou Rewards points. You can then USE these Citi ThankYou Rewards points for free travel.
Read about how to earn and accumulate Citi ThankYou Rewards points here
Read about how to use Citi ThankYou Rewards points here
Read about sharing and transferring Citi ThankYou Rewards Points here
Read about Citi ThankYou Rewards Points expiration information here
Read about what happens to your Citi ThankYou Rewards Points upon death