Why are they kneeling?

"See what happens when Kendrick, our very first Courageous Kid, decides to ask a very important question. Kendrick thinks this Sunday will be like any other Sunday but is in for a surprise when he notices some of his favorite football players take a knee during our National Anthem. Kendrick finds the courage to ask a question and his family and friends find the courage to answer it. Kendrick learns so much and we hope you will too!"--Page [4] of cover.