Your streak is the number of consecutive days that you have completed your daily goal.
For example, a 5-day streak would mean that you have opened the app and completed your daily goal 5 days in a row. If you have built up a streak and then don't complete your daily goal, your streak will reset.
Building up a streak is a good way to keep yourself motivated to learn a little each day and through this spaced repetition, you will allow your vocabulary to continuously build up.