Sigmund Freud broke down the human psyche into three levels: Id, Ego, and Super Ego. These correspond with our mind's reactions to the world around us.
Id deals with the subconscious mind. It is our basic instincts that we have no control over. This is the only part of our personality that is present from birth. The super ego and ego are both results of the world around us, and is shaped as we mature.
Id is the source of our instinctual needs, wants, and impulses, particularly sexual or aggressive drives. These reactions to life are purely from the subconscious and are concerned with pleasure, or the "pleasure principle"—the tendency to seek immediate gratification of any impulse. For example, if you were guided only by Id, you would be likely to sit down anywhere if you're tired, grab food from another person's table in a restaurant if you're really hungry, or walk to the front of the line rather than being patient.
Because this is present at birth, think of how infants cry immediately when they are hungry or uncomfortable. They want immediate satisfaction of their needs.
According to Freud in his "New Introductory Lectures on Psychoanalysis," the Id is "the dark, inaccessible part of our personality, what little we know of it we have learned from our study of the Dreamwork and of the construction of neurotic symptoms, and most of that is of a negative character and can be described only as a contrast to the ego. We approach the id with analogies: we call it a chaos, a cauldron full of seething excitations. ... It is filled with energy reaching it from the instincts, but it has no organization, produces no collective will, but only a striving to bring about the satisfaction of the instinctual needs subject to the observance of the pleasure principle."
The world would be a rather chaotic place if we all reacted solely based on Id, so our psyche developed both the Super Ego and Ego to control that chaos. I will be posting a basic introduction to both of these soon.
The Id is our basic instincts. We want something, we find a way to get it. No consideration of anyone but our own desires.