The eye dropper doesn't require an explanation: select the tool, move the mouse pointer over the favored pixel and click. The color value of the selected picture element is shown directly in the common hexadecimal format (HEX) and also as RGB value via tooltip. You can change the appearance of the color value in the settings and so you can choose whether to include the number sign or not. Also, the color value is shown in an input field: you can simply click and select the color value in order to copy and paste it in CSS.
To ensure that the chosen color value represents the selected pixel under the cursor, a color preview is shown next to the color input field. The preview represents the selected color value in real-time as a square area.