A “roach” is what pot smokers call the end of the joint or blunt, after it has been smoked. Roaches can be disposed of although many smokers prefer to save their roaches to smoke at a later time. By the time you get down to a roach, it is typically covered in resin and might be harder to hit and harder to hold. This is why many smokers use roach clips, something like needle-nose pliers or tweezers to pinch the side of the roach to hold it while smoking. Roaches can be discarded or saved to smoke later. They can be smoked on roach clips, or unwrapped to use the marijuana left inside to add to another joint, blunt, or bowl.
Some smokers like to use a “crutch”, or a rolled up filter that allows them to smoke the entire joint down to the end, which leaves no need to save the roach.