If you look at the slot on the non-drive side crank arm that the fixing bolts go through, there should be a black metal plate. It has a hole on one end that one of the fixing bolts goes through. the other end has a hook that should fit around the other fixing bolt. There is a small pin you can't easily see on the spindle side of the stopping plate. Before putting the crank arm on or taking it off, you have to use something thin to pull the stopping plate up so it protrudes above the slot it's in. If you look at the spindle, there is a small hole in it. When the crank arm is installed, you push the stopper plate down into its slot so its below the surface of the crank arm. In doing so, the small pin on the stopper plate engages the small hole in the spindle. I believe this is intended to keep the the crank arm on the spindle should the fixing bolts and the end cap come loose.
You can figure all this out by looking closely at the install instructions for the crank and crank arm. There is a drawing that shows the plate, the pin and the hole in the spindle and how they fit together when everything is installed correctly.