What is a sysroot exactly and how do I create one ?

I'm working with QT and embedded devices and in many documentation items ( like here) they are talking about sysroots. I've searched the web but I couldn't find anything about what a sysroot is. I have a cross-compiler-toolchain though. What's a sysroot and how do I create one ?

A sysroot is a directory which is considered to be the root directory for the purpose of locating headers and libraries. So for example if your build toolchain wants to find /usr/include/foo.h but you are cross-compiling and the appropriate foo.h is in /my/other/place/usr/include/foo.h, you would use /my/other/place as your sysroot.