A barometer is a device that measures atmospheric pressure. Barometers help meteorologists figure out what the weather is going to be like.
Sure, it's technically a weather man's tool — but you can use barometer more generally to describe anything that can be used to forecast or measure something else. The stock market is a barometer for economic health. Your grades are a barometer of how you're doing in school. For the President, a national poll is a barometer of political success. These kinds of barometers tell which way the wind is blowing — just not literally.