A tech pack is the blueprint for a product. It usually holds front and back flat drawings, a bill of materials (fabric, rib, thread, zippers, labels), a measurement chart for every size with tolerances, construction and stitch notes, colorways, artwork placement and decoration specs, and packing instructions. A revision log records each change.
It matters because factories quote and build from it. Missing measurements or vague notes lead to wrong samples, extra rounds and disputes about what was approved. A clear tech pack also lets you collect comparable quotes and reorder the same product later.
You do not need expensive software to start; a well-organized document with clear drawings and numbers does the job. Ask your manufacturer which format they prefer and which details they need before quoting. The tech pack guide includes a checklist.