A mold, also called a form, is the reusable tooling a precast piece is cast in. Molds are usually steel or fiberglass, built to hold exact dimensions pour after pour, and a plant owns a limited number of each.
That limit is why scheduling matters. You can only cast as many of a piece per day as you have molds for it, minus the time to strip, clean, and set up again. Planning production around real mold availability, instead of assuming unlimited capacity, is one of the things that separates a workable schedule from a wish list.