This is the Brodo mentioned in the corn and pancetta risotto, both recipes from Salt to Taste by Marco Canora with Catherine Young.
I've never made this, but I'm not the boss of your kitchen, and it's technically part of the risotto recipe. All that follows is the Brodo recipe as written by the authors.
Brodo is not stock, but broth. Stock is made from simmering bones and broth is made from simmering meat. The practical difference is that stock contains more gelatin and is therefore an easier starting place for sauces. Because broths are made with whole pieces of meat and poultry rather than bones, they usually have deeper, more complex flavor. Broths also tend to be less cloudy - the protein in the meats in the pot acts as a filter that clarifies the broth as it simmers, leaving you with a beautifully clear liquid.
Brodo is easy to make with ingredients from the supermarket. Start with a chicken. Put it whole in the pot, then add 2 pounds of beef stew meat on the bone and a turkey drumstick (or two wings). Cover the meat by about 4 inches with water (you'll need about 7 quarts in all) and bring it to a boil over high heat.
As soon as the broth boils, begin to "clarify" it, lowering the heat to medium and pilling the pot to one side of the burner so it's partially off the burner. This forces the broth to boil in an oval circuit from top to bottom, circulating all the liquid over and around the meat. As the broth circulates, the fat and other impurities in the broth float to the surface. What will you see when you look into the pot? The broth will bubble along one side of the pot. The rest of the surface will look active but not be bubbling. Fat and scum will rise with the bubbles and settle on top.
Skim every 5 minutes or so. Be finicky about how you do this. Dip the ladle into the broth near the center of the pot just deep enough to barely submerge the front edge. Then keep it still. A thin stream of fat and foamy broth will be drawn into the ladle. Do this a couple times, then wait another 5 minutes and do it again, continuing until the brodo looks clear, about half an hour.
Once the broth is clear, add aromatic vegetables. Chop and then add 2 onions, 1/2 bunch of celery, and 3 carrots. Add a 12-oz can of tomatoes, 1 teaspoon peppercorns, and 1/2 bunch of flat-leaf parsley and simmer the broth until it's flavorful, about 2 hours.
Strain the broth; discard the vegetables but not the meat. (In my mind brodo is forever linked with polpettone, the fried morsels of minced meat that, in my family, give a second life to the chicken, beef, and turkey used in the broth.) You'll wind up with 3.5 quarts of broth that can be refrigerated or frozen.