A single CLAUDE.md that covers everything gets long fast, so imports let you split it into multiple files.
Use @path/to/file on its own line to pull in another file. The path can be relative (@./standards/typescript.md) or absolute. Claude follows up to hops deep, so imported files can import other files.
I find this most useful for teams that want language-specific rules in separate files. For personal project notes that shouldn't be committed, use CLAUDE.local.md. It sits next to your CLAUDE.md and gets auto-gitignored, so your teammates never see it, but Claude reads it every session.