The Buildds are machines that rebuild the archive for multiple Architecture. As the maintainer will build for a single architecture when the upload the package must be rebuilt on the other architectures. This is the job of the buildds. There is a dedicated network of machines for this task.
They try and only have the BuildDependencies of the package installed, so these must be correct for them to build the package.
See http://buildd.net/ for more information.