What is the toolchain upload that happens at the start of every release cycle?
The toolchain is the set of packages which are used to build all the rest, such as gcc, binutils, glibc, etc. They are uploaded at the beginning of the release cycle before other packages, so that all subsequent package uploads are built against these.
The toolchain is the programming tools for that release of Ubuntu. The developers need these tools in order to create programs for that release.