But before you freak out and embarrass yourself, "streaking" probably doesn't mean the same thing to them as it does to you (assuming to you it means "having a few too many refreshing light beers and getting ejected from the stadium for excessive sexiness").
No, streaking on Snapchat has nothing to do with nudity. Instead, it means that two users are engaged in an ongoing "Snapstreak," which is a fancy way of saying they have consecutively opened and responded to one another's snaps. The longer the Snapstreak, the more loudly a person tends to get about the streaking. See, when you spend as much time on the app as Cool Teens™ do, this accomplishment -- denoted by a fire emoji next to each streaker's name, and a number indicating how many days the streak has lasted -- is A Big Deal.