They eat beans mostly, this old yellow pair. (line 1)
Yup, you guessed it: this couple is old. Brooks says so – right in the first line. Like her description of their skin color, this mention of their age immediately colors how we learn about the circumstances of their lives.
Two who are Mostly Good. Two who have lived their day, (lines 5-6)
Brooks's speaker switches away from general description here to channel others' opinions of this couple. Sure, they're good people – but they're done doing any of the important things in their lives. It's a rather devastating assessment, isn't it? We're guessing that this poem functions as a refutation of that opinion: its very existence asserts that there's something worth writing about in these people's lives.
And remembering ... Remembering, with twinklings and twinges, (lines 9-10)
Sure, people with more years behind them have more to remember. It's pretty clear, though, that remembering is the most active and alive part of this couple's life. Their happy times are behind them. The present is more grim – a small room with leftover mementos of a past life.