By default, steps on Nach are one-offs - once they've been completed, they'll become archived and disappear form the Goal Map. However, you can change a step's behaviour and make it repeat by clicking the Edit button on its page, and choosing one of the available types.
Streaks (5) show next to repeating steps, and signify how many times in a row you've completed the step on schedule, without it slipping into the "overdue" to-do list.
When using the shortcut menu to alter a step's due date, you'll notice there are 2 options available: Set next due and Postpone. While these both carry out the same function of changing the due date, the difference is that postpone leaves a negative "missed" action in the step's history, which breaks the streak; whereas, set next due leaves no such mark, so the streak remains intact.
You can see this for yourself with the history chart on the step's page. If a step was postponed, or wasn't completed on the day it was due, a grey "Missed" action will be placed into the chart, which is the point at which the streak resets.
You are free to go back and edit this chart - so there's no risk of permanently losing your streak by forgetting to check things off your to-do list by midnight.