Bluetooth needs to be on for AirDrop to work; the computer should prompt you to turn bluetooth on, but if it doesn't, make sure bluetooth is turned on.
Which devices are you trying to send/recieve files to/from the MacBook Air?
Look through these articles and make sure both devices match the system requirements.
How to use AirDrop with your iPhone, iPad, or iPod touch - Apple Support
Use AirDrop to send content from your Mac - Apple Support
Make sure the orange box is set to either Contacts Only or Everyone. Try using the green box if you are using an older device. My wife has a 2010 MacBook Pro and her computer would not show up unless I clicked "Don't see who you're looking for?"
Similiarly, make sure all devices are updated. If you are trying to AirDrop to someone else's device, they may need to update their software.