This is from breastcancer.org:

Axillary web syndrome (AWS), also known as cording, sometimes develops as a side effect of sentinel lymph node biopsy (SLNB) or axillary lymph node dissection (ALND). Both procedures involve removing just a few (SLNB) or many (ALND) of the axillary, or underarm, lymph nodes. Most people with breast cancer need to have at least one of these surgeries. Scar tissue from surgery to the chest area to remove the cancer itself also can contribute to cording.

If you develop axillary web syndrome, you'll often be able to see and/or feel a web of thick, ropelike structures under the skin of your inner arm. Lymphedema therapists often call these "cords." (In some cases, you may not see or feel the cords, but sensations of pain and tightness will tell you they are there.) You may first notice them when you're doing something that involves raising your arm to shoulder level or above your head. If it happens, cording typically occurs anywhere from several days to several weeks after your surgery, although there have been individual cases where it appears many months later.

With cording, it's possible to have one large cord or several distinct, smaller cords running down the arm. These cords usually start near the site of any scarring in the underarm region and extend down the inner arm to the inside of the elbow. Sometimes they can continue all the way down to the palm of your hand. In some people, cording can extend down the chest wall instead of, or in addition to, the inner arm.

The cords tend to be painful and tight, making it difficult for you to lift your arm any higher than your shoulder or extend the elbow fully. This pain and limited range of motion can have a major impact on your day-to-day life. Cording can be especially problematic if it happens before or during radiation therapy, since this therapy typically requires lifting your arm above your head.

Researchers are still studying what exactly makes cording happen. Some experts believe that the surgery to the underarm and chest area traumatizes the connective tissue that encases nearby bundles of blood vessels, lymph vessels, and nerves. This trauma leads to inflammation, scarring, and eventually hardening of the tissue. This hardening can spread down the fibers of the connective tissue, which causes the cords to form. More research is needed before we'll know for sure what the cords are made of.