What does it mean if I have double vision?

A few possibilities. Double vision can be in one eye alone (monocular) or can be present only when both eyes are open (binocular). Monocular diplopia can be caused by dry eyes, cornea injury, cataracts and other conditions. Binocular diplopia is caused by misalignment of the eyes, and can be seen in strokes, diabetes, aneurysms, strabismus and injury to name a few. See a doctor immediately to determine the cause.
Eye muscle problem. Most often, it means one or more of your eye muscles isn't working properly. Occasionally, it can be the result of a stroke. You should see an eye doctor.