The poem opens with an image of the speaker poised over a page, about to start writing. A pen "rests" between his fingers, implying that he is quite comfortable with writing; at the same time, this pen is "snug as a gun." This unusual simile in line 2, introduced after the caesura's brief "rest," introduces a sense of tension. Guns, of course, do fit well in the hand, and their use, equally obviously, has serious consequences. Perhaps, then, this is subtly arguing that literature has tangible consequences too, and that the writer therefore occupies a position of responsibility.
For now, though, nothing is happening. This is a moment of quiet before activity, suggesting that what follows is partly a meditation on the act of writing itself. That is, the speaker is taking a moment to think about something before he actually puts pen to page. This allows for the introduction in the following stanza of the outside "digging" sound, which will offer another type of work to which the speaker can compare his own.
In part, "Digging" is about being true and committed to what you do—to working hard. Everything about these opening two lines suggests the close relationship between the speaker and his craft. These lines are packed full of alliteration, consonance, and assonance, as though every syllable has been carefully selected by a master craftsman (which, in fact, is true!):
Between my finger and my thumb
The squat pen rests; snug as a gun.
The way in which every sound seems to fit with another (/ee/, /eh/, /uh/, /t/, /m/, /n/, /s/, /g/, /t/, and /th/ sounds all repeat in just two lines!) suggests the way that the pen fits perfectly in the speaker's hand, almost as if writing was what he was born to do. This anticipates the speaker's admiration for his father's ability to "handle a spade" in line 15—both men have a close relationship with their respective tool.