As humidity increases, the tonewoods used in your guitar will swell. And conversely, as humidity decreases, the woods will shrink. Failure to maintain optimal levels of moisture in your guitar can result in many issues, including, but not limited to protruding frets, cracking in the woods, fret buzz, a sunken top, and the bridge may also lift up on an acoustic. The list goes on and on!
The ideal humidity range your guitars should be between 45% and 55% relative humidity. Extended exposure to levels outside of this range can lead to potential issues and/or damage to your guitar.