Specifically within Domain Web Studio the term “supporting articles” are pages under the “category” level. These pages create the 4th tier in the DWS silo structure.
In the image below you will see the dark pink articles are designed to support the silo landing page articles. Supporting articles are generally recommended to have fewer competing pages than the pages they support. Typically you want to think of these pages as segments of the market represented by the silo landing page. For example a silo landing page about bicycle parts would tend to have supporting articles that are about specific bicycle parts; i.e. bicycle tires, bicycle handles, etc.
Alternatively, the supporting articles can be semantically related items such as a 'diet' silo might have a supporting page about 'nutrition'.
The design of a silo structure is such that the silo landing page will generally have significantly more page rank than the supporting articles, which is why it is recommended that the supporting article pages have fewer competing pages; so that they are generally less competitive by nature.