If you see a triangle with an exclamation mark, it means you have selected a color that is out of gamut for CMYK. Below the out of gamut alert, Photoshop displays the closest color that is within the CMYK color gamut.
If you see a small cube displayed, it indicates that the color selection is not "Web-safe." Below the Web safe alert, Photoshop displays a tiny swatch of the closest Web-safe color match. Clicking on either of the closest match swatches will change the selected color to the closest match.
Now take a look at all those numeric entry fields on the right lower quadrant of the dialog box. These allow you to enter numeric color values in a variety of ways. The default for the color picker is to choose colors by Hue. You should have a dot next to the H in the color picker if you are still using the default preferences.
As you move the color picker from left to right in the large color box you are adjusting the saturation of the hue. Try it and you'll notice the numbers next to the S change from 0 (far left) to 100 (far right). Moving the color picker up and down adjusts the brightness of the hue. As you move the color picker up and down notice that the B values change from 0 to 100 as well. To change the Hue, you can click in the narrow rainbow-colored strip next to the large color picker area, or you can drag up and down in this space.
