This is a package for manipulating geometric shapes. S2 is primarily designed to work with spherical geometry, i.e., shapes drawn on a sphere rather than on a planar 2D map. This makes it especially suitable for working with geographic data.
Information about this library can be found at http://s2geometry.io/.
Start with reading the Overview.
Next look at the Quick Start, but this is specific for the C++ version.
The source code is available on GitHub: https://github.com/google/s2-geometry-library-java.