A positive feedback loop or mechanism is when an action causes a reaction, and the reaction causes more of the action, which in turn causes more reaction, and so forth.
An example might be a teacher who assumes that a child is intelligent, and so treats that child as though he is intelligent. The child then acts intelligently because of the way he is treated. This reinforces the teacher’s belief that the child is intelligent, so she acts on this belief with even more conviction, so the child responds even more and so forth.
Generally, a positive feedback mechanism results in explosive growth, and can only be stopped when a limit is reached, or by external means. For example, when the teacher cannot treat the child any better, or the child cannot act any better, or the child is placed in a different class.