Click applies the color under the cursor to the current selection. Dragging a radius calculates the average color of a circular area. If a gradient handle (in the Gradient tool) is selected, that handle gets the color instead of the entire object.
If is pressed, picking the color (with or without , by click or by drag) picks the inverse of the color.
This copies the color under the cursor to the clipboard, as text in RRGGBBAA
format (eight hexadecimal digits).