What is multiple myeloma

This could get complicated so I revert back to one of my life’s philosophies KISS “keep it simple Sid”.
This is a simple explanation from myeloma UK:

Myeloma, also known as multiple myeloma, is a type of bone marrow cancer arising from plasma cells, which are normally found in the bone marrow. Plasma cells form part of your immune system.
Normal plasma cells produce antibodies (also called immunoglobulins) to help fight infection. In myeloma, the abnormal plasma cells release only one type of antibody known as paraprotein which has no useful function. It is often through the measurement of this paraprotein that myeloma is diagnosed and monitored.
Bone marrow is the 'spongy' material found in the centre of larger bones in the body. As well as being home to plasma cells, the bone marrow is the centre of blood cell production (red blood cells, white blood cells and platelets).
In myeloma, the DNA of a plasma cell is damaged causing it to become malignant or cancerous. These abnormal plasma cells are known as myeloma cells. Unlike many cancers, myeloma does not exist as a lump or tumour. Instead, the myeloma cells normally divide and expand within the bone marrow.
Myeloma affects multiple (hence multiple myeloma) places in the body where bone marrow is normally active in an adult, i.e. within the bones of the spine, skull, pelvis, the rib cage, and the areas around the shoulders and hips. The areas usually not affected are the extremities: that is the hands, feet, and lower arm / leg regions. This is very important since the function of these critical areas is usually fully retained.
Most of the medical problems related to myeloma are caused by the build up of myeloma cells in the bone marrow and the presence of the paraprotein in the blood or in the urine. Common problems are bone pain, bone fractures, tiredness (due to anaemia), frequent or recurrent infections (such as bacterial pneumonia, urinary tract infections and shingles), kidney damage and hypercalcaemia.
There have been many new developments in the treatment and management of myeloma over the last few years that have had a significant impact on the way myeloma is treated. Research is on-going to develop new treatments and to use existing treatments in a better, more effective way.
Treatments for myeloma can be very effective at halting its progress, controlling the symptoms, and improving quality of life, but they are not able to cure it. Even after successful treatment, regular monitoring is needed in case the myeloma comes back.

More in depth information can be found:
Here (myeloma UK)
Or here (multiple myeloma research foundation USA)

Glossary:
Antibodies: Naturally produced proteins in the blood that destroy or neutralise specific toxins or infections such as viruses.
Hypercalcemia: Abnormally high levels of calcium in the blood.