A streak is the number of days in a row you have reached your XP (point) goal on Duolingo. Your goal is set when you set up your coach, and can be changed by accessing:
It is represented by a little fire or flame icon next to someone's avatar.
If you are logged in, you can see yours on the blue top bar of the website. You can check if you have reached your daily goal by looking for a little green check mark on the streak icon.
Keep your streak going and see how high you can go! Visit the lingot store and buy a streak freeze with your lingots to make sure you don't lose your streak when you forget to practice for a day. Check back often to make sure it is equipped, as the streak freeze only works once.