What does the concept of cost roll up mean in product costing context?

The purpose of the cost roll up is to include the cost of goods
manufactured of all materials in a multilevel production structure at the
topmost level of the BOM(Bill of Material)

The costs are rolled up automatically using the costing levels.

1) The system first calculates the costs for the materials with the
lowest costing level and assigns them to cost components.

2) The materials in the next highest costing level (such as semifinished
materials) are then costed. The costs for the materials
costed first are rolled up and become part of the material costs of
the next highest level.