Broth or stock is the basic ingredient in soup making, and is also used in many recipes.
You can, of course, make a soup with just water and your soup ingredients. In most recipes, you can simply substitute water (and salt, if desired) for the broth called for. Or, for a meaty-tasting vegan option, add a tablespoon or more of nutritional yeast.
That works just fine, despite what the bouillon cube and soup stock manufacturers would have you believe. Most commercial bouillon or soup stock is mostly water and salt, anyway.
But for an extra nice, rich flavor boost to your soup, you can easily make your own soup stock.
The very easiest way to make stock is to simply take the leftover liquid from cooking meat or vegetables and condense it. You can add salt and other seasonings if you like, or just use the liquid as is.
Condensing the stock is easy. Just put the liquid into a pot and simmer it slowly with the lid off until it's half or less of its original volume. The more the water evaporates, the more condensed and stronger-flavored your stock will be.
This will take several hours, so you can just leave it to simmer while you're doing other things. I usually set my burner at 1 or 2 on a scale of 10 for this. As long as the broth level isn't getting too low, I've even gone to sleep or left the house while broth is simmering on a back burner of my electric stove. A flat-topped woodburning stove is great for simmering things over a long period of time, too.
Once it's condensed enough for your tastes, take the pan off the burner and let it cool. If it's a meat-based broth, you'll probably want to chill the stock and scrape the fat off the top before doing anything else with it. The fat can be saved and used as your oil or shortening in cooking later.
I like to freeze my condensed broth into ice cube trays to use as a substitute for bouillon cubes. After the cubes freeze, I store them in a labeled container and can take out exactly the quantity I need for use in recipes.
Remember that the flavor will be stronger than the original broth, so you'll most likely want to add water if the recipe calls for broth or soup stock instead of boullion. I usually use between 1 and 3 cubes per cup of water.
I like to cook chicken by covering it with water and simmering it until done, so I usually condense and use the water and juices left in the pan this way. Sometimes I do it right away just by leaving the pan on the stove after I remove the chicken. Other times I put it the broth in the refrigerator for a day or so until I get a chance to condense it.
You can make an even more flavorful broth from scratch.
The basic idea is that you simmer meat and/or vegetables in a quantity of water until the flavor and many of the nutritional qualities of the other ingredients have seeped into the water. You can use just about any combination of one or more ingredients for this. Many people like to keep a container in their freezer and dump leftover bits of meat and/or veggies into it until it's full, and then use that to make their soup.
If you are starting a meat-based broth from scratch, it's nice to use the bones if you have them. You can roast them in the oven at 375 or so until browned for a richer flavor, if you like, but that's not necessary.
If possible, it's a good idea to crack or break the bones to allow the calcium to escape into the broth. If you're buying soup bones from the butcher, ask to have them cracked or cut up. Adding a teaspoon or two of apple cider vinegar will help draw the calcium from the bones into your broth.
Put the bones and/or meat into the pan and add enough water to cover them with at least a half-inch of water over the bones. Then add the vinegar, if desired, and whatever spices you want. Anything other than the meat or bones and water is completely optional.
For most broths you can add some salt (I use unrefined sea salt) at the beginning if desired. If you are cooking legumes (beans or dried peas) the salt will make them tough, so you'll want to add any salt toward the end of cooking.
All you do is bring the water to a gentle boil, then turn it down to a simmer so that it's just barely or almost boiling. You want to simmer it for at least an hour or two, but the longer you simmer it the stronger the flavor. Any longer than 12 to 24 hours, though, and you're going to start getting a bitter flavor and degenerating quality. I recommend 1 to 4 hours for vegetable stock and 2 to 8 hours for meat broths.
You may need to add water occasionally to keep the bones or vegetables covered during cooking.
Once you think you've simmered it long enough to get most of the flavor out of the ingredients, remove the meat or vegetables from the water. You can taste them to see if they're getting rather flavorless; if they are it's time to take them out. If you want to use them in a soup, you'll want to cook the broth for a shorter amount of time so that the meat or vegetables still have some flavor.
Now you may want to strain your broth to get out any chunks. Straining is optional, but because you're cooking this for so long, any chunks will not have much flavor by the time you eat them. If you have shards of bone in your broth, straining is not optional. You don't want bone slivers in your soup.
Next, return the liquid to the pot and condense it as described above.
This broth is healthier and much cheaper than anything you'd buy at the store.