What does the "rc" in a npm package version structure

It means it is a release candidate. meaning it is ready to go out, but it isn't proven yet so it's not ready to be marked as a stable release quite yet. You can think of it as a mature beta release.

If no bugs are found and it is deemed (by the developers/community) to be stable it can be marked as a full release.

https://en.wikipedia.org/wiki/Software_release_life_cycle

Generally it means, "release candidate", meaning it's not production ready, but nearly so, and is generally dispensed with the intention that early-adopters/users will file/fix bugs and issues to test the software.