Grub is the second of the Krust-O-Meter ratings. (For more information about the Krust-O-Meter, see here. For more information about the other Krust-O-Meter ratings and other technical details in the game, see here.)
10 points of Grub gets you 1 star, 30 points gives 2 stars, 70 points 3 stars, 120 points 4 stars, and 180 points 5 stars.
Most of the restaurants and food stalls give 10 Grub points each, but there are a couple which give 20.
Which restaurants and food stalls count towards the Grub rating? See below. You can have multiple numbers of the ones marked with an asterisk (*).