In short
A bill of materials (BOM) is the structure that defines all raw materials, semi-finished goods and components of a product, with their quantities and hierarchy.
The BOM is the foundation of MRP calculations, costing and traceability. An incorrect BOM means incorrect purchasing and incorrect costs.
In the process industries the BOM is often called a recipe or formula and may include by-products and co-products. Revision control is needed to track which BOM version each production run used.