Karma is a word from the philosophy of many Indian religions (Hinduism, Buddhism, Jainism, Sikhism). The basic theory is that the universe runs according to certain laws, all described by one word 'Dharma' (Sanskrit) or 'Dhamma' (Pali). The basic theory is: 'cause and affect'. The laws (Dharma) decide what affect is beget from a given cause. Karma is the 'cause' part of this theory. In other words, your actions - mental, vocal and physical are your Karma (plural).
Please note that in the Buddhist philosophy, Dharma does not 'punish' or 'reward' anyone. The concepts of good and bad don't exist here. For example, gravity is one of the laws of the universe, and if you jumped off a building and killed yourself, we wouldn't say that gravity was punishing you.
Selfless service will help 'improve' my Karma.