Stock or bouillion is just flavored water. It can be used for soups and sauces or to poach things. It can be simple and light or intense and heavy. Stock is made from leftover bits of animals or vegetables. It is a way to literally "use everything" from food. Some people keep a stock pot on their stove and add all the little leftover ends of vegetables from the preparation of meals with some salt and just simmer for days.
If you are going to use the leftover chicken that you roasted for dinner, take all the bones and roast it in the oven with some whole onions cut in half, peel and all, a whole garlic with just the top cut off till it just begins to blacken. Then dump it all into a pot with some salt bring to a boil and then simmer for a few hours. You can always add celery or carrot or tomato to add more flavor. Once it has simmered for a few hours (at least 3) you can sieve out all the bones and stuff and then put it back on the heat to reduce to desired strength. You can use immediately, keep it in the fridge up to a week, or freeze in small portions to add to things as needed.
If you are going to make a vegetable stock, you can use the peels of carrots, the insides of tomatoes, the ends of celery, the leftover bits and shells of onions. You can use celery root, parsnips, leek, mushrooms, spring onion, paprika, garlic. You can add herbs like bay leaf, parsley, whole pepper, thyme and rosemary. Water, salt, boil then simmer like the chicken bouillion above.