HC-0X Bluetooth Module Difference between Slave & Master
A slave connects to a master.  A master can be a PC with a Bluetooth radio in it / plugged into USB, or it can be a "master" HC-05 module.

Note the HC-05 can work in either master or slave mode.

Examples of masters are PCs, mobile phones when using bluetooth headsets, etc.

Examples of slaves are bluetooth headsets, phones when connecting to a computer, etc.