Controls how a widget deals with extra space in a single (x or y) dimension.
Alignment only matters if the widget receives a “too large” allocation, for example if you packed the widget with the expand flag inside a Box, then the widget might get extra space. If you have for example a 16x16 icon inside a 32x32 space, the icon could be scaled and stretched, it could be centered, or it could be positioned to one side of the space.
Note that in horizontal context
GTK_ALIGN_END are interpreted relative to text direction.
GTK_ALIGN_BASELINE support for it is optional for containers and widgets, and it is only supported for vertical alignment. When its not
supported by a child or a container it is treated as