BOM is the acronym for bill of materials. A BOM is a listing of the quantities of each of the materials used in manufacturing a product.
Industrial manufacturers are likely to have an enormous number of BOMs. Each of the BOMs will be a very detailed list of all of the quantities of every material used in the various steps of manufacturing each part or product.
To visualize a BOM, think of a bakery that produces only pies. Each pie's BOM will list the ingredients in the pie's recipe. Each BOM will list the number of pounds (or other unit of measure) of the specific fruit, the quantity of a specific sugar (or other sweetener), the quantity and type of cinnamon, the quantity of nutmeg, the type of crust. There will also be a BOM for the pie crust. The pie crust BOM will specify the quantity and type of flour, the quantity and type of butter (or oil), the quantity of salt, etc.