官方解释：

Cartographic projection library (development files)

Proj and invproj perform respective forward and inverse transformation of cartographic data to or from Cartesian data with a wide range of selectable projection functions (over 100 projections).

Geod and invgeod perform geodesic (Great Circle) computations for determining latitude, longitude and back azimuth of a terminus point given a initial point latitude, longitude, azimuth and distance (direct) or the forward and back azimuths and distance between an initial and terminus point latitudes and longitudes (inverse).

This package provides the PROJ development files.

The main purpose of PROJ is to transform coordinates from one coordinate reference system to another. This can be achieved either with the included command line applications or the C API that is a part of the software package.

Coordinate transformations are defined by, what in PROJ terminology is known as, “proj-strings”. A proj-string describes any transformation regardless of how simple or complicated it might be. The simplest case is projection of geodetic coordinates.

我的理解是一个关于 图像映射和坐标转换的库。

官方文档：https://proj.org/usage/index.html