What does streaks mean on Snapchat?

This image appears in the gallery:
Snapchat: Charms, emojis, trophies and everything you need to know

1. 
2. 
3. 
4. 
5. 
6. 
7. 
8. 
9. 

10. 23 images

See the full gallery: Snapchat: Charms, emojis, trophies and everything you need to know