Like Phong said, you will need to turn off your Personal Hotspot before you can AirDrop since it is using the network from your phone. However your MacBook does not need to be on a Wi-Fi network in order to send or receive files through AirDrop. As long as both Wi-Fi and Bluetooth are on on both of your devices, it can send the files without being connected to each other or the internet.
To send things through AirDrop on your Mac, simply right-click on the file(s) you want to send, mouse over share, then click on AirDrop. Then once your iPhone shows up, click send and click accept on your iPhone.
If you would like to know how AirDrop works, you can go here and it explains it in easy-to-understand terms: https://www.lifewire.com/what-is-airdrop-how-does-it-work-1994512