Radial fill is where you fill an object with a color pattern that is circular with the central color merging into an outer color as shown below.
Radial fill
The radial fill control is shown below.
Radial Fill Control
It consists of two separate color users. The top color chooser sets the central color while the bottom color chooser sets the outer color. You can set the position of the central color by dragging on the central color position control.
The central color position selector
Click the 
button to exchange the two colors.