Here is a list of resources you will find helpful:
The Yocto Project website: The home site for the Yocto Project.
OpenEmbedded: The upstream, generic, embedded distribution used as the basis for the build system in the Yocto Project. Poky derives from and contributes back to the OpenEmbedded project.
BitBake: The tool used to process metadata.
For more links, see the "Other Information" section in the Yocto Project Development Manual.