These buttons are located on the toolbar at the top of the screen. Initially they will be "grayed out" (disabled) until you draw and/or select shapes.
Groups selected shapes so that they behave as a single object. i.e. If you move, scale or rotate a group, the transformation is applied to all the items in the group
If you select a group and change the fill or line colour, the new colours are applied to all the items in the group.
However, you can change the fill colour of individual items in a group with 
flood fill tool.
This un-groups grouped shapes and will only be enabled if you select a group