Humidity is measured by a hygrometer and is very important to know during your grow. The more humid the room is, the less resin the plant produces. Also like someone up there said, high humidity is ideal for mold growth, which could ruin your crop either in flowering or after harvest and flush 4 months of time and effort down the drain. Its very important to know what your humidity level is.

The best option is to get a battery powered meter that will run you about $20-30. Mine tells the temp and also has a high/low function where I get the highs and lows of temperature and humidity until I reset it and it also has a wire that I can place anywhere and it will tell me the temp there as well (i.e. the top of the canopy). If you are realy strapped for cash, you can get a small one for reptile cages as a pet store for a couple bucks. Its just a regular old gague but they are usably accurate.