Substack is an online platform that provides tools for writers and creators to publish and monetize email newsletters. It allows individuals to set up subscription-based newsletters, where subscribers can pay to receive exclusive content, often in the form of articles or serialized content. Substack has become popular among journalists, writers, and content creators as a means of direct audience engagement and monetization, bypassing traditional publishing and media channels. It supports a range of content, from personal essays to professional analysis and reporting.