Snow guards are what keep snow from sliding and falling off your metal roof.
Snow guards, or snow breaks, are used to create ice dams and protect your gutters from large amount of snow falling off your roof all at once. They allow the snow and ice on your roof to back up against the guards, then hardening and creating the dam. This means that the snow will melt slowly and small amounts will fall off at a time instead of all at once in a big avalanche-like fashion.