The nub is the little stub of gate plastic left on a part after you cut it from the runner. Every part has at least one; a Gunpla kit can have over a thousand across the build.
Nub removal — flush-cutting the stub, then sanding through rising grits — is the fundamental skill of clean building. A finished kit’s quality is mostly a record of how its nubs were handled.
See it in context in our nippers & cutting tools guides, or explore the tools hands-on in The Bench.