A bill of materials, or BOM, is the structured list of everything that goes into making a product: every raw material, every component, every part, along with how much of each. It is the recipe a manufacturer works from.
An accurate BOM is the backbone of manufacturing operations. It drives costing, how much material a job needs, purchasing, what to reorder and when, and production planning, whether you have what you need to build. Get the BOM wrong and every number downstream, your quotes, your inventory, your margins, inherits the error.
Inside an ERP, the bill of materials connects sales to the shop floor. When an order comes in, the system uses the BOM to know exactly what materials to consume and reserve. That is why cleaning up your bills of materials is one of the most important things to do before rolling out an ERP. The software is only ever as reliable as the product data behind it.