The nrfjprog DLL is a Dynamic-Link Library that exports functions for programming and controlling Nordic Semiconductor SoCs. It lets developers create their own development tools for Nordic Semiconductor SoCs using the DLL's API.
The nrfjprog DLL comes as a 32- and a 64-bit Dynamic-Link Library on Windows. For Linux, it has been compiled as a shared library for both 32- and 64-bit. The macOS variant is delivered only as a 64-bit library. The DLL exports functions for programming and controlling SoCs through SEGGER J-Link programmers and debuggers.