Solid fill is where you can fill a shape with a solid area of color.
For example, the rectangle shown below is being filled with a solid fill of cyan.
A rectangle with a solid fill of cyan
To fill an object first select it.
Now you have two options to set the color:
the first option and the quickest is to click on the desired color in the color bar at the base of the viewport.
The second method is to set the color using the fill properties in the selected objects properties panel.
What you see is a standard AutoTRAX color chooser.
The solid fill control is shown below.
Solid Fill Control