Button hover is what to do with Button Widgets when you mouse over them. You have the option of setting uiHoverMode to determine what to do.
0 = None (no hover)
1 = Image blend uiHoverColor;
2 = Zoom, will Expand button
3 = Glow
Color of the blend when you uiHoverMode=1;
This depends on the uiHoverMode, for zoom it is the zoom factor (ex. =1.1), for image blend it is the alpha.