Within the context of the Yocto Project, QEMU is an emulator and virtualization machine that allows you to run a complete image you have built using the Yocto Project as just another task on your build system. QEMU is useful for running and testing images and applications on supported Yocto Project architectures without having actual hardware. Among other things, the Yocto Project uses QEMU to run automated Quality Assurance (QA) tests on final images shipped with each release.
QEMU is made available with the Yocto Project a number of ways. The easiest and recommended method for getting QEMU is to run the ADT installer. For more information on how to make sure you have QEMU available, see the "The QEMU Emulator" section in the Yocto Project Application Developer's Guide.