OpenWrt Buildroot is a set of Makefiles and patches that allows to easily generate both a cross-compilation toolchain and a root filesystem for your Wireless Router. The cross-compilation toolchain uses uClibc (
http://www.uclibc.org/), a tiny C standard library.
A compilation toolchain is the set of tools that allows to compile code for your system. It consists of a compiler (in our case, gcc), binary utils like assembler and linker (in our case, binutils) and a C standard library (for example GNU Libc, uClibc or dietlibc). The system installed on your development station certainly already has a compilation toolchain that you can use to compile application that runs on your system. If you're using a PC, your compilation toolchain runs on an x86 processor and generates code for a x86 processor. Under most Linux systems, the compilation toolchain uses the GNU libc as C standard library. This compilation toolchain is called the "host compilation toolchain", and more generally, the machine on which it is running, and on which you're working is called the "host system". The compilation toolchain is provided by your distribution, and OpenWrt Buildroot has nothing to do with it.