Sigmund Freud (1856-1939) was a founder of a new stream of psychology, called psychoanalysis. He believed that people might be cured by transforming their unconscious thoughts and motivations into conscious, and gaining an ‘insight.’
Psychoanalysis therapy is about the release of repressed emotions and experiences, thus making the unconscious become conscious. Psychoanalysis is commonly used to treat depression and anxiety disorders.
Freud developed a lot of ideas, which can be combined into one global theory - the psychoanalytic theory of personality development. It argues that personality is formed through conflicts among three fundamental structures of the human mind: the id, ego, and superego.
The id according to Freud is the part of the unconscious that seeks pleasure. His idea of the id explains why people behave in certain ways when it is not in line with the ego or superego. The ego is responsible for creating a balance between pleasure and pain. It is impossible for all desires of the id to be met and the ego realizes this but continues to seek pleasure and satisfaction. The superego, which appears around the age of four or five, represents the morals of society. Freud said that the superego only allows the mind to control its impulses that are looked down upon morally.
There are plenty of works on the topic, which consider Freudian theory from different perspectives. I would recommend these three for a good start