It's a method for keeping track of where your food is when you have low vision or are entirely blind. [Manabu may or may not recall Grey mentioning that he used to be blind during that whole tearful "Rezo used me as a lab rat," confession.]

You think of your plate as the face of a clock with the top rim- [Grey picks up a nearby butter knife and taps the top of Manabu's plate.] -representing twelve o'clock and the bottom rim being six o'clock. So if I handed this plate to you I could tell you that your omelet rice is at six o'clock, your salad greens are at eleven, and you have some tomato slices at one.