A pillar page is a comprehensive page covering a broad topic that links out to detailed supporting articles, each linking back. Unlike a category archive, it is written content designed as a single structure rather than a generated list.
The pillar answers the broad question; the cluster articles answer the specific ones.
Internal links between them are what makes it a structure rather than a collection.
Gaps in the cluster weaken the pillar, so plan the full set before publishing the first piece.