libbrillo is a shared library meant to hold common utility code that we deem useful for platform projects. It supplements the functionality provided by libbase/libchrome since that project, by design, only holds functionality that Chromium (the browser) needs. As a result, this tends to be more OS-centric code.
To help keep the projects in sync, we have a gsubtree set up on our GoB: https://chromium.googlesource.com/chromiumos/platform2/libbrillo/
This allows AOSP to cherry pick or merge changes directly back into their fork.