Snapchat streak

Appears next to the number of days that you and a friend have Snapped each other. If you and your friend don't both send a Snap within 24 hours, you will lose your Snapstreaks.

http://metro.co.uk/2017/09/08/what-is-a-snapchat-streak-6910821/

Critics say this is how Snapchat makes people hooked to Snapchat. People don't want to lose their streak.