Vitamins are dietary components which are required in small amounts to maintain our health. To help the concept stick let's try to understand first where the name "Vitamin" comes from.
It is derived from two words "vital and amine". The meaning of the first word vital is quite obvious which means important. The second word is "amin" comes from a misconception at the time of discovery when it was thought that all vitamins are amino acid in nature which is not true.
So the origin of the name tells you about the function of vitamins - that they are important for the body as they help in the metabolic processes such as vitamin A which is used in the process of vision in the human body.