A "streak" is an uninterrupted succession of active days on Duolingo. So if you do five days and then have a day without Duolingo activity, your streak will be "zero" again.

So people who have reached high levels in their languages, but have short streaks just had a few days off in their time of participation.

it is when you have got your daily goal everyday. if you do not do it one day it starts over.

You can go to the lingot store to buy perks, like ones that can keep you're streak for a day when you're inactive. You get lingots by doing things like completing a lesson. I'm probably going over stuff that you already know at this point so...hope this helped in some capacity