“Stock” is a noun, verb and an adjective. Let´s look at examples of each form.
First, as a noun. Shops “hold” stock (existencias, estoc), or they have something in stock (tener algo en existencias). Shops often carry out “stock-taking”, checking the actual inventory to what the accounts say, as part of the stock control (control de existencias).
Farmers have livestock (ganado) and such farmers are “stock farmers” (ganaderos). There are “stock breeders”, people who raise cattle and horses, for example. Sometimes, lakes are “stocked up” with fish (poblado de peces).
Are you the chef at home? Then sometimes you may “make a stock” (caldo) from vegetables, fish or meat. So there are such things as chicken stock, beef stock, and vegetable stock.
You might have stocks in Santander or Marks and Spencer. So “stocks” in this context are shares (acciones) in a company. You might buy your shares from a “stock broker” (corredor) and he goes to the “stock market” (la bolsa) to make the purchase for you.
Now as a verb. Sometimes people “stock-up” (aprovisionarse), that is, buy a large quantity of something so that there is enough for a long period of time. Let´s imagine that chocolate is going up in price, so you “had better stock up with chocolate before the price goes up” (más vale que haga una buena provisión de chocolate antes de que suba).
Sometimes people say that someone “comes of good stock”, meaning that he comes from a good family (ser de buena familia). Finally, there is a “stock phrase” (una frase hecha).