350. The number may mean several things. 20/350 may refer to your vision without glasses. -3.50 may be your prescription strength, the degree of myopia or nearsightedness. Ask your eye doctor for more details if you are unsure.
Lens correction. It is most likely the power of the lens correction you require. Either + 3.50 (farsighted, hyperopic) or -3.50 (nearsighted or myopic).