Upstream kernel is the kernel directly from kernel.org, the original Linux kernel.
The vendor defined kernel, in this case, the kernel shipped by rockchip/radxa.
Although they are both called āLinux Kernelā their source code are very different. A vendor defined kernel will have support code for a particular hardware that is not yet supported by mainline kernel. The worst part of this situation is that the (rockchip) vendor kernel includes too many vendor-specific hacks and the codebase is diverged from the upstream too much so that almost none of their code in their kernel to bring up the hardware can be directly used in the upstream kernel, and the upstream kernel developers has to implement the drivers by themselves, probably not always from scratch, but still a lot of work.
When the hardware support from upstream kernel lands, we will no longer be locked to rockchipās kernel version and we can use the original kernel for our boards, and we will have our freedom to update our as soon as a new kernel version is released.