Stock is collective noun for the stuff a store or a company has to sell, be it toilet paper, automobiles or clothing. Many stores close once a year for a few days to do an inventory of their stock.
Stock is used in all kinds of contexts, as both a noun and a verb, but always has an eye on the things that are present and available. Stock boys stock the shelves at grocery stores. Consumers stock up on toilet paper. Car thieves take stock (assess the stuff they can do) when they run out of gas and the sirens are getting louder. And finally, companies sell stock — small shares of ownership — in their companies to raise money.