Infection. This is usually a sign of infection. You should see your doctor to get worked up for the infection and treated.
Cyst or lymph node. Lumps in the axillae/arm pit are usually sebaceous/glandular cysts or lymph nodes. The cysts will usually seem to be part of the skin or just below it. The cysts can also turn red and develop a pustular head that bursts, like a large pimple. Warm compress accelerates healing. The lymph nodes are deep, under the skin. Lymph nodes or progressive swelling needs evaluation by a primary care providr.
It is inflamed. The redness tell you have active inflammations going on.
Monitoring Centers. Lymphatic system absorbs/carries things too big to go into the arteries/veins like bacteria eaten by white cells and digested food. Lymph nodes "taste" the lymph for anything bad like bacteria/viruses/cancers, etc. When it detects a problem, it alerts the immune system and your body reacts. They enlarge in response to the inflammation they create when they react. They shrink when all is well.
Several things. Swollen nodes can be due to injury or infection. However, since you have swollen nodes at distant sites in the body, namely armpit and groin, this raises the possibility of cancer such as lymphoma. You should not ignore it and should go see your doctor for an assessment.
If they. Are big enough, yes. Although they can also be high and hard to detect without a cat scan.