Relations between knowledge and understanding, and functions of the latter in development of thinking and in mediation of cognition and of purposeful activities are considered. Understanding is defined as subjective disclosing of the meaning of a knowledge: finding for the latter of a particular functional place in man's consciousness and in the general system of activity orientation. Proceeding from the analysis of notions of communicated (comprehended) meaning and personal meaning a conclusion is made about their real unity in the structure of personality. As a consequence processes of understanding regarded as processes of development of meaning personify interaction between cognition and activity: they make effective the acquired knowledge and at the same time transform thinking.