It simply refers to the environment and symptoms that present themselves to a patient in conjunction and as a result of the cancer.
If someone is determined to die NOT from the cancer itself but from complications of having the disease it can mean a host of things.
A person with cancer, under going invasive treatment (ie: Chemotherapy/radiation), usually becomes weak.
Sometimes this will render a person without an appetite which then progresses to them becoming more and more physically weak from lack of proper diet.
This will be fatal, very often in the elderly and is NOT the direct result of the cancer itself.
The lungs can experience Edema (swelling) and/or begin to accumulate fluid which, by itself, in a weakened state can cause death.
Otherwise can create respiratory problems, pneumonia, and so on.
Lymphedema,.. a swelling of the lymph nodes (the lymph nodes control our immune system).
Hypercalcemia (High levels of calcium in the blood) -- which can effect the heart, etc.
It often goes hand in hand with cancer diagnosis and treatment for anxiety and depression to arise.
Sometimes becoming quite severe.
These mood changes can leave people without a will to live, and will , with other factors, contribute to their decline and eventual death.
Complications of cancer are a conjunction of symptoms all working against the patient that are a result of 'having cancer' and receiving treatment (even post treatment) that can cause death.
For cancer to be the actual cause, it needs to physiologically have caused the death.
IE: Cancerous cells invading organs leading to their dysfunction and eventual death of the patient.
In the end, complications of cancer is , in my opinion, not a very genuine diagnosis of death and the cancer IS the cause of the death ultimately.
However, given how systemic cancer is it just isn't practical to undertake a massive forensic pathological investigation post mortem to try and determine exactly what caused the death of the patient from a pathological viewpoint.