Headlight stickers are, generally, branch logos or a defining symbol of a moped branch that are made of die-cut vinyl and placed on the headlight glass as to not completely obscure the headlight itself. At night the logo shows darkly against the headlight beam.
Headlight stickers are fairly unique to moped culture, and more specifically, the Moped Army.
The decals were popularized by The Decepticons beginning around 1998. Moped Army co-founder, Simon King created the first stickers, showing the Decepticon logo, by photocopying the image onto clear sticker paper and cutting out the logo. Eventually Decepticon member, Jason Nickol got a job working at a sign shop with a vinyl cutter and the club upgraded to the die-cut versions which we see today.
This tradition quickly spread to other Moped Army branches, each with custom branch logo stickers cut for better branch identification.