Solid wood instruments are very responsive to their relative environments. If humidity levels are out of balance your guitar could warp, crack, or incur some type of trauma. Keeping humidity constant doesn't have to be difficult, however it is necessary to maintain the beauty and sound of your guitar for years to come.
The relative humidity (RH) of the air surrounding your guitar should be between 40% and 60%. If you store your guitar on an open stand, your guitar is subject to the humidity of the open air. This is fine if the humidity happens to stay around 50%, but in most cases you will want more control over humidity. Relative humidity can vary indoors just as it can outdoors especially with the use of heat in the winter seasons and air conditioning in the summer seasons. An easy solution is to store your guitar in the case when not in use. The case can slow the rate of change in how the guitar responds to the humidity and can be used in conjunction with a humidifier.
Just as any solid wood guitar can dry out, it can also retain excess moisture and become too "wet." An overly moist guitar will show signs of swelling on the top or back. It can compromise glue joints, loosen bridges, and cause bubbles to appear in the finish. If your guitar is showing signs of excessive moisture, a simple solution is to use a silica gel packet. These can be found at any music store and can be placed in the case with the guitar overnight to reduce moisture.