What does it mean when my bottom lid of my eye is twitching?

Fasciculations. Small, local, involuntary muscle contractions are called fasciculations. They're caused by spontaneous firing of lower motor neurons & can occur in any skeletal muscle in the body. They're commonly encountered in healthy people and are rarely concerning. The cause is most commonly benign. They should subside, but if they continue to bother you, your primary care doc can refer you to a neurologist.
Myokymia "nerves" Small twitching for a few seconds usually of a lower eyelid, or both; but it can also be elsewhere on face or neck; sometimes on a leg. The "beats" only last a few seconds and then are done. My experience with myokymia has been with patients that are overly tired and stressed. Rest usually helps this condition; and it may or may not happen again. If the symptoms are progressive see specialist.
Maybe blepharospasm. Eyelid twitching could be caused by nerve irritation, called blepharospasm, in the orbicularis muscle around the eyelid. Fatigue, dry eyes and other stress factors may play a role. If it is very symptomatic, the overactive nerve firings can be quieted with Botox injections, but often, improved lubrication and resting the eyes from a visually demanding task can be very helpful.
Myokymia. You are describing myokymia - an involuntary twitching of the skin around the eyelids. It's believed to be related to lack of sleep, stress, and caffeine use. Change these risk factors and it should abate.
In most cases, nothi. In almost every case, this is benign and trasient.
Usually nothing. As long as it does not involve the cheek or the corner of the mouth, it is innocent. Can last for up to six weeks- usually, stress, anxiety, lack of sleep and will usually go away in the same mysterious way it began. If it is both eyes, involving the cheek, or longer than six weeks, go to the ophthalmologist.
Eye irritation. Usually nothing. Sometimes its caused by eye irritants or dry eyes, so using artificial tears may help. If it lasts more than a day, see and eye doctor...
It's too dry. This is usually due to dry eye, blepharitis or "dandruff" of the eyelids or it could be due to an undiagnosed thyroid condition. Stress and long hours at the computer are other possibilities. If using over the counter eyedrops doesn't help you should see an ophthalmologist.