The clock icon with text "Delay" allows you to pick a delay until the Picking Tool becomes active (in seconds). This is useful if you need to prepare an action like hovering your mouse onto a button in a program before the Picking Tool starts.
Ensure you see the program you wish to read a pixel from on your screen and press the "Pick" button. Your screen will freeze and turn whitish. Now you can select the pixel you want by hovering your mouse over it and simply clicking once.
For similar instructions please search for information on the Windows Snipping Tool as the user interface, workflow and keyboard shortcuts were inspired by it.