It's a children's rhyme that you say as you're walking along the sidewalk, trying not to step on the line that divides the segments of sidewalk.

Thanks for the kind explanation, Luke!

The full children's rhyme is, "Don't step on a crack, or you'll break your mother's back." There isn't any real meaning behind the second part of the sentence. It's only there because "back" rhymes with "crack." When I was a child and we would go outside during school hours, we would always play this game. As you walk, you have to try not to step on any lines or cracks in the sidewalk/pavement. It made walking more entertaining for us.