Between my finger and my thumb The squat pen rests; (1-2)
Right away we get an idea of who, or at least what, the speaker is (a writer). So that's something, right?
My father, digging. I look down (5)
And right away, we know what occupies his mind. The speaker's identity is totally shaped by how he sees his father.
Bends low, comes up twenty years away (7)
Now we can see further into the past, to where the speaker's identity began to take shape as a young boy, helping out his old man in the potato fields.
Loving their cool hardness in our hands. (14)
Check out this sensory detail. Doesn't it just stick with you? No wonder the speaker remembers it to this day.
The cold smell of potato mould, the squelch and slap Of soggy peat, the curt cuts of an edge Through living roots awaken in my head. (25-27)
Again, the speaker has very vivid and specific memories about potato digging that stay with him into the present. Our guess is that in his future career, he's going to be doing a lot of writing about potatoes.
But I've no spade to follow men like them. (28)
So it turns out our speaker is quite different from the other men in his family. Why does that matter so much?
Between my finger and my thumb The squat pen rests. I'll dig with it. (29-31)
By the end of the poem, the speaker identifies as both different from his father and grandfather (because he's a writer, not a manual laborer), and similar to them (because his work requires that he dig, too).