Self-care has a very simple definition. It is the things we do, or avoid doing, in order to look after ourselves, physically and/or mentally. If you have a car, you need to maintain smooth operation with regular servicing and gasoline/electricity. If you want to travel safely in this life, you also need to care for your mind and body.
Self-care maintains our health and wellbeing so we can function and enjoy our life.
Many people confuse self-care with selfishness and roget that we never say our car is selfish when it needs service. We accept it and are even willing to pay for it.