Mailboxes per doorstep is the number of mailable delivery points that share a single physical entrance. Mail is priced and delivered per mailbox: a twelve-unit building is twelve recipients, and each one gets a card. Knocking is priced per doorstep: that same building is one place to stand. The ratio between the two is the gap between what a mail campaign reaches and what a person on foot can reach, and it varies from 1.0 to over 9 across U.S. cities.
Read the full definition paper: The Cost of Knocking 200 Doors: How Far You Actually Walk, Measured Across 1,200 U.S. Cities