French words can sometimes intimidate fresher home cooks, which makes sense because part of the French aesthetic is being intimidating. Luckily, making a roux—aka flour cooked in fat—is once of the easiest chef-like maneuvers one can master.
One can get very technical when making a roux (and I have), but you don’t have to. Flour provides the starch (which is what thickens), and the fat coats the flour to make sure it doesn’t clump up when it hits the liquid. Just combine a roughly equal amount of flour and fat in your pan (an equal amount by volume is fine for butter; use mass with other fats), and cook it. If you want a luscious, golden gravy, use butter as your fat and cook the flour until it smells toasty and looks blonde in color. If you want a darker gravy with a more “roasted” flavor, use oil or lard (butter will burn) and cook your flour until it is a dark chocolate brown.
Keep in mind, however, that the longer you cook your roux, the less thickening power it will have, so make a little more if you’re planning to use a dark roux. For turkey gravy, I usually aim for somewhere between the two extremes, and cook my flour in butter until it is tan in color with an extremely pleasant, nutty, toasty aroma.