A stock is a flavored liquid made by simmering bones, meat, seafood, and/or vegetables in water. Stocks are the foundation for many soups, sauces, and gravies. The document discusses the ingredients and preparation methods for different types of stocks, including white stocks, brown stocks, fish stock, and vegetable stock. Proper preparation such as blanching bones, skimming fat, and gentle simmering is emphasized to extract flavor while avoiding cloudiness.