Imagery is something an author includes in order to help a reader create a mental image of what is being described. Most imagery appeals to one of the five senses: taste, touch, sight, sound, or smell. In regards to Seamus Heaney's "Digging," much imagery is used.
First, Heaney appeals to the sense of touch by mentioning his finger and thumb. An engaged reader may begin to rub his or her finger and thumb together to gain a sense of the initial image provided here.
Next, Heaney offers readers a simile. Comparing a pen to a gun, readers can sense what either, or both, feel like. Those familiar with holding a gun can picture the weapon in their hand.
The third sentence appeals to the sense of hearing. The "rasping" sound of the spade hitting the rocks is relatively familiar. While not all readers may be familiar with a spade, most are familiar with the sound metal makes when it hits rock.