When you redeem a reward, your points are immediately exchanged for the reward. But, you do not need to use the reward immediately. If you choose not to use the reward, it will be saved in your panel under Your rewards.
To use a previously redeemed reward, login to your account. Then click Your rewards and select the reward you want to use. Copy and paste the discount code at checkout, or click the Apply code button to apply it to the cart immediately.