The Pleasure Principle is a psychoanalytic concept developed by Sigmund Freud. To understand the Pleasure Principle, you must first have a rudimentary understanding of how Freud looks at the human mind and personality.
Freud divides our minds into three parts: the id, the ego, and the superego.
The id is the part of our mind that reacts to need and desire. We are hungry, we want to eat. The ego controls the id so that we act within the acceptable standards of society—we don’t take food away from someone else just because we are hungry at the moment. We wait until it is appropriate for us to eat. The superego is able to look at right and wrong—it houses our conscience. The superego might make it possible for us to actually give our food to someone else, even though we are hungry ourselves, because we believe it is the right thing to do.
The Pleasure Principle, as you may have already surmised, is the part of the id that seeks to immediately satisfy our physical desires. We want to feel pleasure or relief, so our first instinct is to simply take an action to find that satisfaction. If the ego and/or superego judges that this action is not socially acceptable or morally right, it prevents us from taking this action until a more appropriate action can be found.