Non-maleficence means to do no harm. Traditionally, this is at the heart of medical ethics and is part of the Hippocratic Oath (an oath that new doctors take in many countries). An example of a non-maleficent action would be stopping a medication known to be harmful or refusing to give a medication to a patient if it has not been proven to be effective.
However, ethical dilemmas often occur. In many medical situations, non-maleficence must be balanced with the principle of beneficence (an action done for the benefit of others). For example, many beneficial medications may also have serious side effects and so the risks and benefits must be carefully considered by doctors and patients. Ultimately, the patient must decide whether the benefits outweigh the risks before consenting to a treatment.