A runner is one channel of the sprue — the thick rail that carries plastic to a group of parts during molding. Kits label each runner with a letter tab (A, B, C…), and instructions call out parts as a letter plus a number: part C-14 is piece 14 on runner C.
Learning to read runner letters is the single biggest speed-up in early building: you stop hunting through every frame and go straight to the right rail.
See it in context in our kits & grades guides, or explore the tools hands-on in The Bench.