Click anywhere in the color picker to the right. This will select a color. After you select a color, you'll notice the RGB, HSL, and hexadecimal values are shown for your selected color.
Here's an explanation of hexadecimal and RGB.
RGB stands for Red Green Blue. Therefore, when using RGB notation, you are specifying how much red, how much green, and how much blue should be included in the final color. This is because all colors are a mix of these three colors.
It's the same with hexadecimal notation, except, the notation uses base 16 to specify the red, green, and blue values.
For example, if you use RGB notation, rgb(255,0,0) specifies "all red and none of the other colors". This is because 255 is as high as you can go in RGB notation.
You could of course, use less red and more of the other colors, for example rgb(72,61,139) results in DarkSlateBlue.
In hexadecimal, ff is as high as you can go and 00 is as low (i.e. none). Therefore, the color "DarkSlateBlue" would be represented as color:#483D8B. You could have, of course, represented this color by its color name (i.e. color:DarkSlateBlue).
