It is a blogging site, but it’s also notable for being in favor of free-speech absolutism, to the point of allowing Nazis on the platform. From Wikipedia:

In January 2022, the Center for Countering Digital Hate accused Substack of allowing content that could be dangerous to public health. The Center estimated that the company earned $2.5 million per year from the top five anti-vaccine authors alone. The three founders responded via blog post affirming their commitment to minimal censorship.

Substack faced further criticism in November 2023 for allowing its platform to be used by white nationalists, Nazis, and antisemites. In an open letter, more than 100 Substack creators threatened to leave the platform and implored Substack’s leadership to stop giving bigotry a platform. In response, Substack CEO Hamish McKenzie said the company would continue to allow the publication of extremist views because attempting to censor them would make the problem worse. Creators like Casey Newton, Molly White, and Ryan Broderick left the platform as a result.

Yeah it’s a blog platform that has a variety of functionality built in, including their big one: newsletter-style email subscriptions. Everything can be optionally paywalled by the blogger.