In general, a spotter is someone trained to look for or "spot" something.

Weather

In particular, skywarn spotters are frequently used in the U.S. to report severe weather. These volunteers are trained by the National Weather Service, and given a phone number or amateur radio frequency to call if a major weather event, such as a tornado or severe thunderstorm occurs where the spotter is located.

Weight training

In weight training, a spotter is a person who stands by the person lifting the weight, in order to help them should they be unable to complete the exercise and secure the weight properly. Spotters are also used in powerlifting, but they are not required for weightlifting.