In biology, a stimulus is defined as some detectable change in the environment which causes a living organism to respond to that change. An example is sunlight in the environment allowing plants to photosynthesize, which creates their food and creates fresh oxygen for us to breathe.
Stimuli are why animals, plants, and all living things are able to respond to their environments and surroundings. For humans, an example of a stimulus is cold air in the environment, which causes us to shiver and maintain homeostasis in our bodies so we do not freeze.