Normally you put a camera onto a tripod by attaching the camera body right to the tripod head itself. But if you have a really heavy lens this is a bad idea. Large lenses can often weigh far more than the heaviest SLR cameras. So the right way to do it is to attach the lens to the tripod head via a tripod mount. The camera then sort of hangs off the back of the lens unsupported, but this isn’t a problem - lens mounts are designed to handle that kind of weight easily.

Lens tripod mounts or collars are mounting rings with built-in clamps. These rings make it easy to rotate the camera from portrait (vertical) to landscape (horizontal) position. If the maker of your lens sell a tripod mount for the lens then it’s probably wise to get one.