Lymphoedema is swelling caused by a build-up of lymph fluid in the surface tissues of the body. Following treatment for breast cancer this lymph fluid build-up may occur as a result of damage to the lymphatic system because of surgery and/or radiotherapy to the lymph nodes (glands) in the armpit (axilla) and surrounding area. Sometimes it can be caused by cancer cells blocking the lymph system.
If you think you have developed lymphoedema, it is important to contact your breast care nurse or specialist team for an assessment. The sooner you are treated, the better the outcome may be.
Source: Breast Cancer Care UK
I found videos on YouTube of Jeanette Zucker, Clinical Specialist and Physical Therapist, explaining the cause, risk and management of Lymphoedema to be some of the most concise and straight forward explanations available online.