An Acronym for Dynamic RAM.
This is RAM which, if not "refreshed" (ie recharged, which can be done by reading it), loses its state after a short while. Therefore, some other circuitry has to continuously keep reading through memory over and over to preserve its state.
This is one of the reasons that make DRAM comparatively slow. However, it achieves high packing ratios, ie a lot of storage capacity in little room and is therefore cheap. A computer's main memory is therefor comprised of DRAM.