Positive feedback is a type of feedback.
Open systemss (ecological, biological, social) contain many types of regulatory systems, among which are systems that involve positive feedback and its relative negative feedback. Positive and negative do not mean imply desirability. The negative feedback loop tends to slow down a process, while the positive feedback loop tends to speed it up.
When a change of variable occurs in a system, the system responds. In the case of positive feedback the response of the system is to change that variable even more in the same direction. This has a de-stabilizing effect, so does not result in homeostasis. In some cases (if not controlled by negative feedback), a positive feedback loop can run out of control, and can result in the collapse of the system.
Positive feedback is used in certain situations where rapid change is desirable.