Den Strip or Patrol Emblem...

This patch indicates the number assigned to the Den for which a Scout is a member. All Scouts and Leaders wear this emblem as part of the uniform.
 
Second year Webelos Scouts (and Leaders) who's Den has chosen to utilize the Patrol for identification would wear the Patrol patch instead. There are numerous Patrol identities available.

These emblems are worn on the right sleeve directly below the American Flag.