"The Bean Eaters" opens with a description of a lonely couple, worn by age, eating their usual meal of beans. The watching third-person speaker calls this couple an "old yellow pair," an image that evokes oxidized paper or a tattered old pair of shoes. This couple seems to have weathered a lot together.
By calling the couple a "pair," the speaker signals that these two are very close. The members of a "pair" are made for one another, and each is incomplete without its other half. (Think of a pair of headphones or a pair of pants.) The couple's closeness suggests that they've shared a long and loving relationship. But perhaps that they've grown so closely together as they've aged that they've lost their separate identities.
The couple's meal—and the poem's title, "The Bean Eaters"—suggests that this "old yellow pair" doesn't have much money to spare. They're not just eating beans for dinner tonight, they eat them "mostly." This cheap, plentiful food will become the poem's central metaphor for the couple's poverty.
But while this couple isn't rich, the sounds in these first lines suggest that they're not too unhappy, either. Mellow, assonant long /oh/ and /ee/ sounds in "They eat beans mostly, this old yellow pair" ease the audience into the scene; there's certainly something sad in this repeated meal of beans, but also something gentle. And the strong end-stops in these first two lines evoke the couple's unhurried pace. It's as if they're pausing to catch their breath as they move around their room, getting dinner ready.
There's also a touch of humor in the poem's second line: "Dinner is a casual affair." Readers might expect to see "Dinner is a casual affair" on an invitation to a cocktail hour, not as a description of an elderly couple eating beans at home. The speaker here appropriates upper-class language to ironically highlight the couple's poverty. The lighthearted mood of this line has an edge, creating a tension between an upper-class "casual affair" and the stark reality the couple faces.