explain a negative and a positive feedback. and taking one example for each of them?

Homeostasis is the maintenance of a constant internal environment (in your body). Negative feedback exists to maintain homeostasis. This means that if an external event causes the internal environment to change, a negative feedback mechanism occurs to keep the internal environment stable.

For example, if the ambient temperature increases (i.e. it gets hotter outside), your body temperature may rise, which would cause harm to your cells. To prevent this harm occurring, you will start sweating to maintain your body temperature by evaporative cooling. Conversely, if the ambient temperature decreases, you will start shivering to generate heat to keep your body temperature stable.

Positive feedback mechanisms in the body are rarer, and serve to push the internal state further from the stable normal levels. The only example I can think of is during birth, the mother requires internal positive feedback to be able to deliver the baby safely.