This image shows the plume that was raised when the LCROSS booster rocket struck the Moon in a region that is permanently shadowed from the Sun. The mission's goal is to search for the water-ice that might be frozen in these shadowed regions, since such ice would be a valued resource for any astronauts that might return to the Moon.
There are two likely sources for this water-ice. One is the solar wind, which can implant hydrogen into the lunar soil, which would then combine with the oxygen in soil to make water. Another source is comet impacts, which can deposit water as well as other volatiles that can then freeze out in these permanently shadowed regions. Since the spectra collected by LCROSS also reveals other volatiles, such as methane, ethanol, ammonia and carbon dioxide, all of which are known to exist in comets, cometary impacts are now a favoured theory for depositing water on the Moon.