The gate is the narrow neck of plastic that connects a part to its runner. It is deliberately thinner than both the runner and the part, so it is the designed break point — the only place your nippers should ever touch.
Good cutting technique is really gate technique: first cut a few millimeters out from the part to relieve stress, then flush-cut the remaining stub. Cutting tight to the part in one squeeze pinches the plastic and leaves a white stress mark.
See it in context in our kits & grades guides, or explore the tools hands-on in The Bench.