A finger drawn beside a passage catches the modern reader’s eye just as effectively as a highlighted sentence. Often called a manicule, this mark makes a place in a book easier to notice. The reason it was noticed is a separate question.
The Folger’s illustrated essay Adages and Annotations shows several forms of these hands in an early printed book. Looking at those examples is a useful introduction before reading catalogue descriptions of marginal marks elsewhere.
Try a close-looking exercise with one of the documented examples. Locate the fingertip, then the nearest line or section. Does the hand identify a short phrase, an entire paragraph, or an area whose boundary is unclear? Write down the smallest passage you can confidently associate with it.
Next, separate observation from interpretation. “A hand points beside this passage” is an observation. “The reader endorsed this teaching” needs more evidence. A reader might mark a difficulty, a memorable phrase, or something they planned to question.
If words accompany the drawing, read them before assigning a purpose. If the mark is undated, do not automatically attribute it to the first owner of the book.
This approach is especially valuable for religious texts, where attention can too easily be mistaken for adherence. A mark records an encounter with a passage; its meaning depends on the rest of the evidence.