I am using a projection that I have defined from a WKT format, and not included in QGIS' default roster of projections. See below in the image. The origin of my projection is 40N and 100W. Why is the cross hair centring where it is? Or is this just a generic default that occurs when you define your own custom projection in Q?
The cross simply indicates where your map canvas is centered right now and the violet box shows you the current extent of the map canvas. Normally, there should be a second, light red box that shows the extent the selected CRS is valid for.
Screenshot: zoomed to Europe, the small violet box shows the extent of the map canvas (=red outlined frame), including the point the canvas is centered on, somewhere in eastern Poland/western Ukraine. The red box covers the whole world, because I selected EPSG:3857 (Web Mercator) in the CRS dialog window.