The sprue is the molded plastic framework that a kit’s parts arrive attached to — the thing you lift out of the box that looks like a plastic window frame full of pieces. Molten styrene flows through it during injection molding, so every part hangs off it by a small channel.
Cutting a part off the sprue is the first act of every build, which is why the first cut has a whole technique (and this site) named after it. Most builders use “sprue” and “runner” interchangeably; strictly, the runner is one channel of the sprue.
See it in context in our kits & grades guides, or explore the tools hands-on in The Bench.