"They eat beans mostly, this old yellow pair". This line refers to two old people who eat beans most of the time, and that's maybe because they are old and don't have enough resources or energy to make any other type of food.
"Dinner is a casual affair". This line makes it look like they are very used to each other, therefore, dinner has become a routine of eating beans with each other's company.
"Plain chip ware on plain and creaking wood, tin flatware". Here, the author describes items that seem to be in their house for a very long time and as well used for a very long time on the dinner table.
"Two who are mostly good. Two who have lived their day". With this line, the writer describes the couple as an old one again. A couple who has been good most of the time during their long lives and that have lots of experience in life.