Curator, Ann Temkin: Three Standard Stoppages are three lines that Duchamp made by holding a thread above the ground, and dropping that thread to the floor. The thread was a meter long, but of course, instead of falling on a perfectly straight line that would then be a meter-long line, it ripples a little. You could try the experiment yourself.
Duchamp decided, why not use that readymade standard of measurement achieved by dropping a thread to the floor as a new authority? What's inherently better about a straight meter?
So what matters here is the role of chance. And Duchamp, in a way, is opposing the fortuitousness of chance to the boredom of the received idea of a standard, measured length. And he's saying to the viewer, "Look at the possibilities."