This telescope, located as the name suggests at the south pole is designed to look for small changes in the Cosmic Microwave Background (CMB) caused by the influence of clusters of galaxies. The south pole was chosen to site the telescope because the cold, dry (little moisture in the air) conditions are perfect for astronomy at the sub-millimeter wavelengths being used by this telescope.
The telescope will make use of an effect known as the Sunyaev-Zel'dovich effect to search for large clusters of galaxies. The SZ effect occurs when photons from the CMB interact with energetic electrons found in clusters of galaxies, some of the CMB photons are boosted in energy by the electrons through the Compton effect. By accurately measuring the CMB you can see regions where the CMB appears to be slightly hotter than it should be, these regions generally correlate with the position of clusters of galaxies. It should be noted that the CMB (see pic below) naturally has fluctuations in its temperature, these were the "seeds" that led to the formation of structure in the Universe after the Big Bang, so to determine which fluctuations in temperature are due to SZ and which are due to inherent fluctuations is actually slightly tricky. There are also other effects that need to be taken into account but you get the picture, its pretty hard.
By mapping the distribution of clusters it is possible to learn something about the elusive Dark Energy, particularly it may be possible to determine between the two competing explanations of Dark Energy, the Cosmological Constant and Quintessence.