The eyedropper (pipette icon) allows you to sample from anywhere in the Blender window. The eyedropper can be used to select different kinds of data:
This is the most common usage, the eyedropper is used to sample a pixels color from anywhere within Blender.
Dragging the cursor over the window to sample a line which is converted into a color ramp.
This is used with object buttons (such as parent, constraints or modifiers) to select an object from the 3D Viewport or Outliner.
- Camera Depth
Number fields effecting distance can also use the eyedropper.
This is used to set the camera’s depth of field so the depth chosen is in focus.
E will activate the eyedropper while hovering over a button.
LMB dragging will mix the colors you drag over, which can help when sampling noisy imagery.
Spacebar resets and starts mixing the colors again.