A topic cluster is a group of related content pieces that revolve around a single "pillar" page. The goal is to cover a broad topic in-depth to establish topical authority.
Internal links help search engines understand the relationship between your content. They distribute "link equity" and keep users on your site longer by offering related reading.
Pillar pages are typically 2,000–4,000 words. They should provide a high-level overview of the entire topic, while cluster pieces dive deep into specific sub-topics.