What is the streak message, and will it upset a regular who misses a week?

A streak congratulates a customer for consecutive weeks with a delivered order, at milestones you set, with 4, 8 and 12 weeks as the defaults. Nobody is ever told they broke a streak; the message only exists for the milestone. It ships off under Marketing → Loyalty & stamps, along with the anniversary note and the savings statement. It is a way to celebrate a customer without discounting to them. The Loyalty & rewards tab in the docs explains streaks, anniversaries and savings statements.