Final answer:
Active reading is the process of engaging with a text through margin notes and underlining key points to improve comprehension and retention, unlike passive reading which involves skimming without deep engagement.
Step-by-step explanation:
The act of underlining key points and writing reminders or suggested actions in the margin when reading incoming mail is called active reading. Active reading involves engaging with the text by making notes, which could include summarizing plots or chapters, asking questions, and writing personal reflections or objections in the margins. This method contrasts with passive reading, where the reader may simply skim the text without truly engaging with the material, thus reducing comprehension and retention.