To recap, plasma cells are in the business of producing the components of immunoglobulins, a.k.a. antibodies. These proteins are composed of chains of molecules, two "heavy" chains and two "light" chains each. There are two types of light chains, distinguished by the names "kappa" and "lambda". Normally, heavy chains and light chains are produced in more or less equal proportions, and assembled into antibodies. Light chains that have not been linked up with heavy chains -- "free" light chains -- are left floating around in the blood, and eventually are excreted in the urine. Too many free light chains of either type, or the wrong ratio between the two types, is taken to be an indicator of possible myeloma activity. In my case, the lambda number is normal, and the ratio is just under the top end of the normal range. The reason the ratio between the two types is significant is that there are conditions other than myeloma that can cause elevated numbers, but unlike myeloma, these conditions typically affect both types equally.
The oncologist is not yet worried, because this could be a one-time anomaly. Day-to-day variations in free light chain numbers can be substantial, affected by a number of factors, including for example hydration. To be considered truly significant, similar results would have to be repeated for successive tests. Beyond this, the numbers based on this blood test are not considered definitive; a much more accurate test, called "24-hour urine protein electrophoresis", would be carried out in order to justify taking any remedial action. I took this test not too long ago, and passed. So there is no cause for panic just yet.