The ARM Embedded Toolchain is a GNU licensed cross compiler for ARM CPU architectures, allowing you to compile C/C++ code into binaries which can execute on CPUs such as the Cortex-M line of microcontrollers.
When dealing with C/C++ embedded development, you have the choice of several compilers. Keil is probably the most well-known commercial brand in the embedded toolchain space. They have their own proprietary compiler and IDE which is quite popular for commercial applications (it's pricey and only runs on Windows).