It’s a short-lived (usually 1 day), short length video or photo on your account that any of your connections can view as part of a carousel of other stories. You can post yours and/or view your connections’ stories in sequence.

I.e. You go into the app. 10/100 friends have posted stories. You watch them one after another (without needing to go to each friends account) to see what they were up to for the day.