Its usage has been described in a previous post: selecting adb device.
Basically, when you are dealing with several devices or emulators, instead of specifying the serial number on every command line you can do something like
$ adb $(android-select-device) shell
1) 02783201431feeee device 3) emulator-5554
2) 3832380FA5F30000 device 4) emulator-5556
Select the device to use,
to quit: 1
uid=0 gid=0@android:/ #
I hope you find it useful.