It's called a hygrometer, and u can get one at walmart with the thermometer right on it for $6. humidity generally aids in keeping moisture within the plant and soil, in other words, low humidity will dry the plant and the soil out quickly, usually meaning u have to water more often than normally. Too high of humidity on the other hand, is a breeding ground for bacteria and fungi, not good either. humidity should be around 50% for veg, 40% or lower during flowering.