Marcus's high school has installed gait recognition cameras to identify and track students who leave the classroom or the school without permission. These replaced facial recognition cameras, which had been ruled unconstitutional.
Marcus tricks the cameras by putting gravel in his shoes to change the way he walks. Later in the book, he has sprained his ankle, and opts not to add gravel to his shoes to fool gait recognition cameras in the city because his foot is sprained, which also alters his gait.
Cory Doctorow, the author, describes gait recognition in some detail in keeping with his didactic aim of teaching young readers to repurpose technology and fight surveillance and control.