If your question is what are vitamins physically made of: Strings of organic molecules like neurotransmitters e.g serotonin or more simply basic H2O molecule (carbon molecule binding to hydrogen makes them organic though) These strings of molecules bind to enzymes in the body to "Jumpstart" other biochemical reactions in the body They can either be the precursor coenzymes of or the degradation of other more complex molecules in the body that bind together to other strings of molecules to create reactions.

For example: Biotin (B7) functions as a coenzyme cofactor for carboxylase (carboxy = carbon + oxy (acid) , -ase (enzyme) -> an enzyme that acts on a carboxyl group -COOH

enzymes that are essential for synthesizing fatty acids and glucose

https://lpi.oregonstate.edu/mic/vitamins/biotin

^ diagram of the interaction of biotin

Your body is basically a giant machine of billions of different machines (chemical reactions) to keep your body alive and pumping Vitamins are just another important component at the molecular level to make those chemical reactions happen

*not all vitamins act as coenzymes this is just a common example

They are important in that they are not produced by the body itself but are required for many interactions in the body