The scene selection panel allows you to manually change the Preview and Program scenes in OBS. The Live Switch buttons handle most of the preview and program changes automatically, so you'll likely only need to use this panel if something's gone wrong, so it's very handy to have just in case.
The drop down panel lets you choose the scene you want to change to. The current preview and program scene will have "(Preview)" or "(Program)" written next to it respectively. Select the scene from the list and click on the Change Preview or Change Program button to change it.
This panel is greyed out if NodeCG doesn't have an active connection to OBS. Check that OBS is running, and check if you're connected on the OBS Connection panel.