Oscar,
Positive nodes, are one of the indicators used to stage many cancers. A lot of emphasis is put on nodes, but they are not the "be all",but are very important when thinking about stopping further spread and what should be removed during surgery.
The lymphatic system is where some cells go to die and some cells are produced. Brian, Gary or anyone who wants to jump in if you have a better explanation or something to add, I'm sure Oscar would welcome the info.
If you think of the lymph system as the cardiovascular system's sewer then this is basically what it does. Immune cells and other body protective mechanisms are not as active in this system. So a cancer cell that is not brought into the lymphatic system bound by an immune cell to be eliminated can lodge itself or invade other tissues. To simplify the lymphatic system it parellels the blood vascular system but is not as smart. If a mutagenic cell happens to take up residence and reproduce in a lymph node it can be transported to any other area in the body via this system.
Swelling of one node due to reproduction of a malignant cell does not mean that all the cells that were produced in this node by the cancer cell stayed there. Think of the lymph system as the toll highway or short cut through the body. There are less cops to regulate the law breakers.
Positive nodes give the doctors a heads up that un-restrained cells have free reign to go where ever they want. A positive node means that the bodies immune system couldn't keep up with the need to eliminate the problem.
Removing other nodes in the same chain allows doctors to, hopefully cut off the transportation of other bad cells that could not be controlled and left the overwhelmed lymphnode.
Salient lymph node testing is a method that oncologists use in some situations to detect wheter or not cancer has spread beyond a positive node. Prior to removal of the positive node a radioactive marker is injected into and around the node. Then after a presribed waiting period usually a few hours a scan is done that will show the doctor where the dye has travelled. If other nodes are positive then they and other nodes should be removed during surgery. It is a way of figuring out the potential for how far a cancer has spread and what needs to be done to "cut it off at the pass" so to speak.

I hope this is useful.

Mike