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The actual front page has "The Package Manager for Everywhere" as its subtitle (and that exact description as part of its <title>), and that is basically all you need to know. It's a staple for Unix-y power users on macOS (but supports Linux too, including WSL), and version 1.0 will be ten years old next week. You're not very far off from complaining that Postgres release notes don't explain that Postgres is a database.
The second header on the main page:
What Does Homebrew Do?
Homebrew installs the command-line tools and applications you need across macOS, Linux and WSL.
Homebrew is the Mac equivalent of APT, Pacman, RPM, BSD ports, etc. It's a fairly traditional package manager for software that doesn't come with macOS. "brew install postgresql" installs Postgres into /opt/homebrew, for example.