Yikes!
Apple created AirDrop on the iPhone so you can easily transfer photos, videos & other files. However, some creepy dudes have found another use for it...to send women unwanted pics of their manhood.
Yes, really.
If a pervert is nearby and you have your Bluetooth on with "Everyone" able to access you via AirDrop, you could be the recipient of some unwanted photos.
To prevent this from happening you should either keep AirDrop completely off or put it on "Contacts Only."