Fill: need an explanation of the bottom left corner

At the bottom left corner of the screen there is two items - Fill: and Stroke: - that show the colors for selected objects.

Some of my objects aren't coloring. Fill: at the bottom has the left half for the bar grey checkerboard, and right half shows the color I selected. No color is showing in the object.

What does the split in the color mean?

How is it changed?

How can I get my object to fill with color?

Thanks.