make sense of index terms in dl.acm.org website
Similar to what is done at other professional societies that publish journals, the ACM has a classification system for submitted papers. Authors classify their own submissions. The system is hierarchical to provide a bit of granularity.
Potential readers use the assigned tags to find articles relevant to their own interests, and the editors use them to assign area editors and reviewers.
The tags used on this site are a bit similar in terms of finding questions related to a readers interest.