MULTIPLE MYELOMA QUERY

I have a close relative who just found out that he has multiple myeloma and as I understand it has to do with cancer in the blood, and the number of red and white blood cells. I was told that it was found in the very, very early stage and if the M.D. (a hemotologist) had not been so thorough it would not have been discovered. He has been referred to Mayo in MN and he is waiting to get the date.
Can anyone explain in simple terms what this disease is and anything that you might know about it