You can use this tool to pick a color or a texture — click on the corresponding position in the 3D view (the active material channel will be taken into account) or in the Texture view. Click with the left mouse button to define the color/texture as the foreground color, or right-click to define the color/texture as the background color.
This option loads the color you click on into the Color manager.
If you enable this option, the texture you click on is loaded into the Color manager. You can paint this texture using a paint tool. Read here.
If this option is enabled, the color you click on in the Texture view or 3D view is picked. With the option disabled, the color in the active layer is picked — this may be completely different from the composite color you see in the Texture view. Therefore in general, keep the option enabled.
This option applies to Solid Color mode only. Rather than pick the color of the exact pixel you click on, this option picks the average color of all pixels within the specified radius. You can enter values from 0 to 100 pixels. For example, if you click on the border between a black column and a white column with a large Sample Radius, the color picked will be a medium gray.