Take a look at the example on the left side of the challenge page. Use that example to help you write the code that follows the test instructions on the bottom of the challenge page.
The @keyframes does the following: colorful is the animation name, which can be referenced by the object you want to animate. 0% {background-color: blue} tells the animation object to start with a blue background. 100% {background-color: yellow} tells the animation object to end with a yellow background.
The code will automatically transition from 0% to 100%.