No Leak Waterbox... Basically, a radiometer is an instrument that measures natural blackbody radiation, usually coming from a specific location (you can use optics and things like that to "look" at a specific place). Satellites commonly have radiometers that measure the "temperature" of the earth in various frequency ranges, for example. Blackbody radiation is a function of physical temperature and frequency (or wavelength), and it is calculable from Planck's law: http://en.wikipedia.org/wiki/Planck%27s_law
That's all great, but nothing is really a "true" blackbody: http://en.wikipedia.org/wiki/Black_body
It turns out that all objects have an "emissivity" that is not quite equal to 1. It is always less than 1. If an object's emissivity were 1, it would be a real blackbody, and you could calculate its blackbody radiation from Planck's law.
This is all basic background. The problem that exists in the real world, is that you have to calibrate the radiometer somehow, in order to know what absolute temperature it is measuring. That is, you need to provide the radiometer's input with a KNOWN amount of blackbody radiation (that is, power, in Watts), so that when the output says (for example) 1 Volt, you know what that 1 Volt corresponds to. This is very difficult, since, as I said, no objects are actually true blackbodies. If I just had a true blackbody, I would put it in front of the radiometer, and at a specific temperature and frequency, I could say "Thanks to Planck's law, I know exactly how much blackbody radiation it is emitting into my radiometer (say, 100 mW). The radiometer now says 5.3424 Volts, so I can deduce my radiometer's calibrated sensitivity in Volts (output) per blackbody Watt (input)."
The point of the "Water Box” or “ABC source" pictured above, is that it uses the near-blackbody property of water, by putting it into this special shape, so that multiple reflections from the water surfaces add up in a special way, such that the entrance to the ABC source approximates a true blackbody, in a very specific and exact way.
It is a little more complicated than that, but the basic point is that the ABC source allows calibration of radiometers in a certain frequency range, which was not previously possible.
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