Your streak counts consecutive days on which you completed and claimed at least one day of a course.
Extending it
Finish a day's lessons and claim them. Opening the app is not enough — the claim is what registers.
Breaking it
Miss a day without cover and the streak returns to zero. It does not decay gradually, and there is no partial credit.
What it is for
Fifteen minutes a day beats a weekend binge, and the streak exists to make the daily habit easy to see rather than to punish you for a bad week. It is a score, not a currency: it cannot be sold, transferred or exchanged, and it has no value outside the app.
If you are going to miss a day
You can hold a streak freeze in advance, or repair a broken streak shortly after it breaks. Both are paid for with XP and are covered in Streak freezes and repairs.
Travel and time zones
The streak follows your device's clock. Crossing time zones can make a day look shorter or longer than it feels; it does not break a streak by itself.