QC.EBU.IO basically is a database of definitions of Quality Control tests for audio-visual material. It also hosts reference and example test material (audio and/or video sequences).
You can read this specification.
The tests are used by media companies to verify content, for example when exchanging material with other parties, or when migrating archives.
The QC.EBU.IO collection functions as an industry reference and helps ensure different QC tool implementations give comparable results for the same Tests. This is important, as providers and users of audiovisual material may not use the same QC product. It also helps educate media staff on what can go wrong with material/what errors to look for.
Besides hosting the collection of existing QC Test Definitions, QC.EBU.IO is also used as an authoring tool to create new QC Test definitions. This is a collaborative effort, taking place in the EBU QC Group.