SDL stands for Simple DirectMedia Layer library. SDL is a cross-platform graphics library that provides low-level access to graphics, audio, keyboard hardware via OpenGL and Direct3D. SDL is well-ported to Windows, Mac OS, Linux, Android, and iOS. 

SDL is written in C, works with minimal effort with C++, also there are many bindings available for other languages like C#.

Simple SDL program example to explain rendering and keyboard handling

FreeRTOS is a free and open-source real-time operating system (RTOS) that runs on most of the modern microcontrollers, including STM32. In 2017, Amazon took control of the FreeRTOS project and now offers regular maintenance and support.

An operating system (OS) is software that manages other software and hardware resources in a computer system. You may be familiar with the most popular universal operating systems such as Windows, macOS, Linux, iOS, and Android. Universal OS is often designed to focus on the user experience.

A module definition DEF file (*. def) is a text file containing one or more module statements describing various attributes of a DLL. The DEF file is especially useful when one needs to disable the name mangling of the exported functions and variables.