Discoverable mode is a state within Bluetooth technology integrated devices that enables Bluetooth devices to search, connect and transfer data with each other. Discoverable mode is used to propagate the availability of a Bluetooth device and to establish a connection with another device.
Techopedia explains Discoverable Mode
Discoverable mode is primarily used in Bluetooth powered mobile and cellular devices. Typically, it is only activated for the initial connection.The other device in turn scans for all devices that are in discoverable mode. It can see the device name of the available device and requests a connection. If discoverable mode is turned off, no other device can search or connect to that device, even if the Bluetooth is activated.