With iOS 6 Apple introduced a feature located in the lockscreen that gives you more options when receiving a call, aside from the usual "answer" or "decline". By now you know that from the lockscreen you can swipe up from the bottom right corner of the screen, where a camera icon is located, to go to the camera app. You can now do a similar gesture when you receive a call, but instead of a camera icon, you will see a phone icon.
When you swipe up when receiving a call you have two new options: "Reply with a Message" or "Remind Me Later". "Reply with a Message" gives you the option to send a text to that person that you can't talk right now. There are three default messages, but you can add your own message by going to Settings > Phone > Reply with a Message.
The other option given on the lockscreen is "Remind Me Later", which essentially reminds you later that you couldn't take that call. After you choose this option, you select when you want to be reminded.