Understanding WB

As the others said. Keep in mind that there's nothing that actually measures the quality of the light. The camera sensor just records the light that hits it.

So to get a neutral-looking result, compensation is needed, according to certain algorithms.

Also note that the numbers in Lightroom will probably not exactly match the numbers in camera. That's normal. They are two different processing engines, and may require different values to achieve the same visual result.

As Conrad says, the color of the scale is indicative of the correction you're applying. If your image is too yellow, meaning the color temperature of the scene was low in the Kelvin scale (like a sunset, or candle light) then you can compensate for that by adding blue to the scene, which will complement the yellow tone and balance the white colors to a neutral. Likewise, if your image is too blue (has a higher color temperature) you compensate by adding yellow. 

So, if your original "as shot" white balance is set to 5500k, and the actual scene you shot was 2000k, it will show up with a strong yellow cast, and as you move the WB slider to the left, to lower the temperature, you're adding blue to offset the yellow cast. Move the slider and the image will become increasingly blue. That's why the slider is colored that way - it's a cue as to what will happen to the image when you move the slider in that direction. Move left, more blue. Move right, more yellow. 

All the sliders that affect color in Lightroom are cued in this way, so you can quickly know what will happen to the image when you move the slider. Youkll see this in Saturation and Vibrance, as well as all the color mixing sliders. 

The explanation I’ve heard is that the white balance adjustment is not the amount you are setting it to, but the amount of correction that’s being applied.