DAAC is an acronym for "Distributed Active Archive Center," and PO.DAAC is the acronym for Physical Oceanography DAAC. As components of the Earth Observing System (EOS) http://eospso.gsfc.nasa.gov/
Data and Information System (EOSDIS) https://earthdata.nasa.gov/
, DAACs are institutions that generate EOS standard products and carry out NASA's responsibilities for data archive, distribution, and management. Many of these sites also carry products that pre-date EOS.