Humidity is just the percentage of how much water is in the air. The higher your humidity the less water you’ll need to provide as the leaves will absorb the water molecules in the air. Some growers will even feed their plants this way by spraying their plants, and interestingly enough, it appears that a plant’s stomata won’t close even at night if humidity is high enough.
Optimal range should usually be 40-60%. Optimal humidity for seedlings and plants in veg won’t be as critical as later stages, but low humidity may lead to the stomata closing even when the lights are on, which could damage the plant’s photosynthesis process.
Open stomata are what allow the plant to take in gases, such as carbon dioxide and oxygen. Carbon dioxide is needed for photosynthesis, while oxygen is needed for respiration. Stomata close at night to protect from water loss (plants lose water when their stomata are open), but this doesn’t need to happen if there is a lot of water in the air.
Flowering: 50% and less (The lower the better)
Drying and Curing: 60% on the money.
A low humidity in veg isn’t the worst thing. You’ll want to make sure you’re watching your watering since she’ll dry out faster. Your plant will probably be a little more fickle if she has low humidity in veg.
There is also a relationship between humidity and temperature. If your temperatures drop, but the vapor content in the air stays the same, your humidity rises; if temps rise then humidity drops. This is obviously the opposite of what we want to happen since we want both lower temps and humidity in flowering.
Since this relationship that we don’t want occurs, I highly recommend a dehumidifier. Raising humidity is not terribly hard because you can just lay some pots filled with water in your grow tent. That way when the water evaporates it says in your tent. Be careful to not have any loose wires and things like that around since you have standing water.
Some growers attempt to get their humidity as low as possible the last week or so of flowering to increase trichome production, but I don’t know if that helps or not.