A base theme is a parent theme that your custom theme inherits from. By declaring a base theme in your info YAML file, you get all of its templates, libraries, and behaviour without copying code.
You then override only the parts you want to change. This is a huge time saver compared to building everything from scratch.
Popular base themes include Stable9, which provides a minimal foundation, and Radix or Bootstrap Barrio for projects using specific CSS frameworks. Olivero itself can serve as a base theme if you want to start from something close to production ready. Choosing the right base theme matters because it shapes how much customisation effort your project will need. Pick one that aligns with your design goals.