A nail is used to vaporize your concentrate as opposed to regular bowls that are made for tobacco and dried herbs. Nails are available in different materials - mostly quartz (crystal), ceramic, glass and titanium, where the last one can take the highest temperatures. You start by heating them up to your preferred temperature, thereafter you put the concentrate on top and inhale. If you take bigger tokes it's worth it to complete your setup with a carb cab to increase the efficiency of your rig. It's important to remember that glass nails shatter if you heat them too much, so be careful!