The manual option allows one to set a specific color temperature number. Yes, color has a temperature which is expressed in kelvin-degrees. This is not a measurement of how hot the source of light is, rather a representation of the color temperature. The hotter the temperature is, the bluer the color will be; the cooler the temperature is, the warmer the color. This option involves dialing in an educated guess of what the color temperature is. You can memorize the kelvin chart for about 6-8 specific situations, or carry a cheat sheet. Personally, I prefer neither as I am bad at memorizing. I am more technical with how I go about these sorts of things, so I carry an 18% grey card. It is great for giving me an indication for the right exposure in any lighting condition as well as expressing what the color balance adjustment should be in the given light. All I need to do is shoot a picture of the grey card, set it as my white balance indicator to match the grey on the camera, then go on to shoot the rest of the images in the session.
Does this technique really make that much of a difference in the long run? Well, it depends how you shoot. As long as you are shooting in RAW, it does not matter what color balance you preset. You can always do the corrections during post-processing. Just make sure not to go over the top and try to stay true to the natural hues and lighting. If you happened to shoot JPG’s, not RAW, you can still do “faux” color adjustments to correct white balance errors when shooting during post-processing, it just won’t be quite as good.