A topic cluster is a set of articles that cover one subject from different angles, connected to a main "pillar" page with internal links. The pillar gives the overview; each cluster article goes deep on one question.
Why it matters
Covering a subject thoroughly, with pages that link to each other, helps search engines understand what your site is about and which page to rank for what. It also keeps readers moving through your site.
Example
A pillar page on "home espresso" could link to articles on choosing a machine, grinding for espresso, dialling in a shot and cleaning, each linking back to the pillar.
What to do
Pick a few core subjects your customers care about, plan one article per distinct question, and link related articles together with descriptive anchor text.