According to Wikipedia:
Acting out is a psychological term meaning to perform an action to express (often unconscious) emotional conflicts. The acting done is usually anti-social and may take the form of acting on the impulses of an addiction (ie. drinking, drug taking or shoplifting) or in a means designed (often unconsciously or semi-consciously) to garner attention (ie. throwing a tantrum or behaving promiscuously).
The action performed is usually destructive to self or others and may inhibit the development of more constructive responses to the feelings. The term is used in sexual addiction treatment, psychotherapy, twelve-step programs, criminology and parenting.
Acting out painful feelings may be contrasted with expressing them in ways more helpful to the sufferer, e.g. by talking out, expressive therapy, psychodrama or mindful awareness of the feelings. Developing the ability to express one's conflicts safely and constructively is an important part of impulse control, personal development and self-care.