If it's the same as brown sauce in the west, then probably a sauce made of worcestershire and ketchup. Also possibly just worcestershire or other fish based sauce.
The bottle just says "sauce", but it's implied to be a very common kind of Japanese brown sauce made of various fruits and vegetables. It's a bit like Worcestershire sauce but thicker and a bit sweet. Perhaps you can compare it more to HP Sauce or barbecue sauce than what the west would consider proper Worcestershire sauce. Anyway, the picture isn't more specific so just "brown sauce" it is.
What the Japanese call "sauce" (ソース) is theoretically medium viscosity Worcestershire sauce, but it's a lot more complicated than that. For starters, Japan's version of Worcestershire sauce is not actually anchovy or fish based. And depending on who you ask, it's either vegetarian Worcestershire sauce or brown sauce. What kind of "sauce" you get can vary by region as well...