[Analog24]

It's essentially a way to look up meta data about the different data files produced by the CMS detector. There are petabytes of data produced by the detector and these are stored in countless data file. In order to determine which datasets are available and right for your particular analysis you would use the DAS system to look for them and find out where they're located. This is a complicated task b/c the petabytes of date are distributed across the CMS computing grid that spans many dozens of institutes across the globe.