Basically, a toolchain is a bunch of development tools used in a chain (the output of one tool feeds into the input for the next tool, hence a toolchain), usually in order to compile source code (such as ROMs or kernels). Developers often use modified toolchains that can include optimizations for specific architectures focusing on performance or stability. However, some developers may not want to use some of the highly modified toolchains.
To that end, XDA Recognized Developer and Forum Moderator The Flash is posting regularly updated prebuilt, unaltered GCC and Linaro toolchains that are built and tested on Arch Linux and Ubuntu 17.04.
Check out the GCC and Linaro Prebuilt Toolchains