Well, humidity is the amount of moisture in the air measured from 0% (no moisture) to 100% (raining or fog in the middle of the summer in temperate regions)

If a room needs a certain humidity, get an inexpensive humidifier at a store like a drug store (one that you'd put by your bedside if you've got a bad cold), a fan and a hygrometer. A hygrometer can be obtained at a pet store and is used to take readings of temperature and humidity for reptile cages. Get a digital one. Dial ones are hard to read and are less accurate.

The fan helps circulate the humid air throughout the room.

Also, something called perlite (the little white bits you sometimes see in plant soil) holds moisture very well and if you're growing in a very small room like a closet, get a small basin and some perlite and drench the perlite with water but don't drown it. Just let it soak it up and dispose of the excess water. Give it some heat to release the moisture into the air.

RO water is run-off water. It's rain water that you'd get from your gutters from your roof or something. It's more pure than tap water and more preferred by plants.

HTH