Concepts are reusable definitions for statistical concepts like frequency, currency and reference area (country), and are contained in a Concept Scheme. When defining a Data Structure Definition, each Dimension, Attribute and Measure is given a specific Concept. Concepts can be coded by linking them to a specific Codelist or left un-coded in which case they take value that conforms to a specific data type or Representation.
The SDMX Cross Domain Concept Scheme provides a range of standardised Concepts maintained by the SDMX Statistical Working Group (SWG) and can be viewed on the Global Registry site https://registry.sdmx.org/items/conceptscheme.html.
Using standardised Concepts wherever possible is good practice because it enables harmonisation of statistics i.e. it’s easy to compare statistics produced by different organisations if referenced concepts mean the same thing.