Choose the target of the adb related command when you have more than one device or emultor....

1. look up the connectd device

$ adb devices

and you can see the result like described as below and the each line represents a serial number of each device or emulator

List of devices attached
HT837GZ03715 device
emulator-5554 device
emulator-5556 device

2. the symantic usage is

$ adb -s <> - directs command to the USB device or
emulator with the given serial number

for example :

$ adb -s emulator-5554 shell

$ adb -s HT837GZ03715 shell