Bill of materials ..it's kind of materials list.
Assume that u want to manufacture a computer.
computer contains lot of components..
so under computer this all component materials comes..
again among them if u take CPU part...it further drill down to some more materials...
This is called ..Bill of Materials...
In SAP this wud maintained in tables about this part materials info for a particular material.
u can check MAST table for some clarification.
Hope u r clear about this term