It's the metal 'stand' that you bolt down onto the slabs, and to which you fasten the uprights of the greenhouse.

It is four lengths of metal channel that you fix together in a rectangular shape and then fasten this to the ground. The greenhouse sits on top of the frame. The fixtures for fastening the base to the ground will be part of the pack.

I 'think' we're saying the same thing, Mr. MB.... ;-)