If the indicator light is flashing on your Apple Bluetooth device
When your Bluetooth enabled Apple wireless keyboard, mouse, or trackpad is turned on, the indicator light flashes twice in succession, and then repeats (blink-blink-pause, blink-blink-pause). This means the device is discoverable. When a Bluetooth enabled device is discoverable, other Bluetooth devices can detect or connect to it. After three minutes, the indicator light turns off. The device is still on, but it is no longer discoverable. To make it discoverable again, turn it off, then turn it back on.
When the Apple Bluetooth device connects successfully with your Mac, the indicator light glows steadily for 3 seconds before it turns off.