If this option is selected, the Android emulator user interface will be displayed on screen during the build.
Untick this option if you do not want the emulator to be visible while it runs.

Disabling this is useful, for example, on Linux machines that do not have a graphical user interface.
If you do require the emulator to be displayed on such machines, you can install and enable the Xvnc plugin.
With Xvnc enabled for the build, you can keep this option selected and the emulator will be launched in a VNC session.