This is a project setting that controls the number of audio channels you want to use in your project. The default is 2 channel stereo, which suffices most people. You can choose 6 (5.1) for surround sound, but Shotcut does not yet support a variety of surround channel layouts.
For 6 channels, Shotcut uses the FFmpeg default channel layout for 6 channels:
Front Left, Front Right, Front Center, LFE, Surround Left, Surround Right