The bits stored in DRAM remain readable for a time (sometimes minutes) even after the power is cut. In normal operation frequent refresh is needed to avoid decay the decay doesn't happen nearly as fast as the refresh cycles make it seem. Cooling the cells lengthens that time, from minutes up to hours (depending on the temperature), permitting an adversary to read them without much time pressure.

Paper on that: http://citp.princeton.edu.nyud.net/pub/coldboot.pdf