This tool may be used to fill an area with a specific color. You can position the cursor on the area that you wish fill with color and then click the left mouse button to fill with the primary color, or click the right mouse button to use the secondary color.
The region that gets filled is defined as the area surrounding where you clicked that has a uniform color. The tolerance of this definition of uniformity is controlled with the tolerance slider in toolbar.
The paint bucket will also use the fill style that you have chosen in the Toolbar.
In the example shown below, two circles were drawn. The Tolerance was set to a value of 75% to make sure that the anti-aliased (softer) edges of the circles were less affected. The paint bucket was then used three times: once inside each circle, and then once in the area between the circles. A different color was used for each use of the paint bucket.