Most joints that are rolled in European countries contain tobacco. The tobacco is added for the taste, but also to make a joint burn better.
The tobacco sometimes leaves tar on the lips and fingers of the smoker. To prevent this, a small piece of cardboard (often torn from the pack of rolling paper) is placed at the mouthpiece of the joint. This 'mouthpiece' is also called roach, tip or clip, but the exact name is different for each country and place. The advantage of a roach is that you can smoke the entire joint because you can hold the joint with your fingers at the roach.
A joint consisting of the roaches from other joints ensure an awesome smoking experience.
In America, joints are not always rolled with tobacco. This also differs per US State and place, but smokers usually do not put tobacco in between. In America, the word joint is also not widely used, but blunt and spliff are. What is called a roach or tip in Europe is called a filter in America. The use of a filter is conceived for the same principle because otherwise the fingers of the smoker would be burned.
A ''roach"' is the leftover of a blunt or joint after it has been smoked.
The beauty of a filter (or roach or tip) is that resin sticks to it. And that resin contains a lot of THC. With each inhalation, the THC smoke brings a little bit of that resin to the roach, making it very strong. That is why roaches are often collected to later smoke the resin in a new joint. This gives you a joint with a very strong effect.