A topic cluster is one main page on a subject plus pages that each answer a question around it, all linked, so Google can read them as a group.
A topic cluster is one main page on a subject, plus several pages that each answer one question around it, all linking to each other. This site's Search Console module is one: a main guide and six lessons, each linked to the others.
Two reasons it helps. Google finds and weighs pages through links, so pages that link to each other are found sooner. And Google's AI features run several related searches behind one answer and cite across them; in an Ahrefs study of 863,000 results, 31% of AI Overview citations ranked between 11 and 100 for the main search.
Each page in a cluster should answer a different search, or two of them will compete: see keyword cannibalisation. The evidence is under "Related pages link to one another" in what affects Google rankings.