There are a lot of articles on Free Light chains you can find via google.
Search for "multiple myeloma and free light chains".
Here is one good article:
https://my.clevelandclinic.org/health/diagnosti...

My opinion/explanation: Multiple Myeloma is all related to cancer cells that clone themselves and create excessive number of protein cells in your bone barrow. These cancer cells crowd out the normal protein/red blood cells/white blood cells/immunoglobin cell and damage your bones with hypocalcification and damage your kidneys with excessive protein cells. These are classified as light chain cells and there are so many of them that they cannot find enough large chain cells to connect to and are thus counted as free light chains in your blood test.They can be Kappa or Lambda light chains. So this is one of many ways to help identify MM cancer cells from a blood test.
These cancer cells are clone themselves as Immunoglobin cells. So your blood test may show a large number of either IgG, IgA or IgM cells. When I was first diagnosed, my IgA count was 3464 and the normal range is 47-310. So that stuck our like a big red sore thumb in my blood tests which led me to a bone marrow biopsy. BTW, I would be classified as IgA Multiple Myeloma. The cancer cells cloned themselves to replicate IgA cells.
Then the bone marrow biopsy is the definitive test to determine if you have MM. They are counting the number of abnormal protein cells (which are the cancer cells) in your bone marrow and determining a ratio (or percentage) of abnormal to normal protein cells. My understanding is anything above 3% is classified as MM. I was lucky. Mine was 25%. But many people have had 80-90% and have had successful treatment and survival from MM.
Research this as much as you can via google and ask your medical team for al the information they have on blood testing. Remember, blood testing are symptoms or markers that provide information. They do not provide definitive proof of a disease.
Good luck and don't let all these technical terms, tests, weird names, etc. scare or get you down. And remember; MM is treatable and survivable.

Thanks for this question and the helpful responses! My MM protein count at diagnosis was 80%, and I am still here (age 73). There are many drug therapies available and new ones in the works, so I am always hopeful. It's definitely not an easy road, but each day is precious and I am grateful to be alive.

Thank you that was a great explanation for me. I could understand that, you’re right about all the terms being hard to understand. Great help, thanks !

Kris, I'm sorry to hear about your husband's relapse and bone pain. I hope this is not new bone damage. Great to hear he is responding well to treatment. I pray he gets back to MRD soon.
Good luck and keep us informed of his progress.

My Husband has relapsed at 90% he has a lot of bone pain but is responding well the second time too.