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Pour 1/2 cup (120 ml) of soy sauce, 1/2 cup (120 ml) of rice vinegar, and 1/4 cup plus 4 teaspoons (64 g) of sugar into a small saucepan. If you don't want to use rice vinegar, you can substitute dry sherry, sweet marsala wine, or dry white wine. Turn the heat on to low and stir the sauce until the sugar dissolves. Let the sauce bubble gently and simmer it until it's as thick as you want it to be. For example, for a thin sauce, turn off the heat as soon as the sugar dissolves. For a thicker sauce, simmer it for 10 to 20 minutes. Let the eel sauce cool completely before you pour it into a squirt bottle or storage container. Drizzle the eel sauce over your favorite sushi, cooked noodles, or grilled meats. Store the eel sauce in the refrigerator and use it within 5 days.
Measure the ingredients into a pan. Stir and simmer the sauce. Cool and use the sauce.