Not meat grease. It's made from pan-drippings when meat is baked or roasted and you skim the grease off before making the gravy.
So you'd start with a couple tablespoons of melted butter, then add in some flour and cook it until it thickens. This is a roux. A roux is often cooked on low heat until it turns color to add more flavor to the gravy. A roux can be almost white to a brick color. Skimmed pan drippings are added to the roux and it's heated until the roux dissolves and the gravy thickens.
You can also thicken gravies/soups by making what is called a slurry. You heat your pan drippings to almost a boil and then add in some starch(fecula in Spanish) which has been mixed with some cold water. This is added to the drippings to make a quick gravy. This is much easier, but it is not as good as when a roux is used.
So you'd start with a couple tablespoons of melted butter, then add in some flour and cook it until it thickens. This is a roux. A roux is often cooked on low heat until it turns color to add more flavor to the gravy. A roux can be almost white to a brick color. Skimmed pan drippings are added to the roux and it's heated until the roux dissolves and the gravy thickens.
You can also thicken gravies/soups by making what is called a slurry. You heat your pan drippings to almost a boil and then add in some starch(fecula in Spanish) which has been mixed with some cold water. This is added to the drippings to make a quick gravy. This is much easier, but it is not as good as when a roux is used.