robot_localization is a package of nonlinear state estimation nodes. The package was developed by Charles River Analytics, Inc.
Please see documentation here: http://docs.ros.org/noetic/api/robot_localization/html/index.html
EDIT for ReZoom: Include paths in /include/navsat_conversions.h and /include/navsat_transform.h is changed from the original repository (This was done to bypass an error). Make sure you edit them for your system.
This repository requires both the apt installation of GeographicLib and an installation from source in your home repository (This will be the include path you have to change as mentioned above).
Run sudo apt-get install libgeographic-dev ros-noetic-geographic-msgs to get the apt installation (if not done already). Next, install from source, go to your desired directory (I keep it in my home) and run:
git clone git://git.code.sourceforge.net/p/geographiclib/code ./geographiclib
cd geographiclib
mkdir build && cd build
cmake .. && make
sudo make install
You should be able to build this package without any errors after this.