This folder contains crates that are effectively libraries developed for and used by Tock. The libraries are standalone, have become reasonably stable, and are likely useful outside of Tock. Therefore they have moved to the libraries folder as separate crates so that external projects can leverage them.
Someday these libraries could become their own repositories if the need arises.
To use one of these libraries, simply add them as a dependency in a Cargo.toml file. For example:
[dependencies] tock-registers = { git = "https://github.com/tock/tock" }
Cargo will handle finding the correct folder inside of the tock repository.