Bones define the type and flavor of stock.
Meat stock takes its flavor from bones so, if we use bones from beef, we get beef stock, from chicken, chicken stock, from fish, fish stock, etc.
Bones are rich in collagen which is broken down into gelatin by heat.
Gelatin is the substance which gives stocks a relative thickness and causes them to solidify at low temperatures.
Vegetables and herbs imbue the stock with added flavor and aromas.
By boiling all these ingredients together we achieve a tasty liquid at the end of the process.