What does clean on board bill of lading mean?

It means that the product wil be load without dirts or damages!!

All cargo when loaded must be loaded in the condition described in the contract, any difference, deviation (poor storage,packing,handling etc causes dirt, damage ) from the stated condition. This difference is normally noted as a remark in the Bill of Lading. As almost all banks will not pay out if the Bill of lading is "dirty", shippers request a "clean bill of lading" ie one without any remarks. However if the condition of the cargo is unacceptable , It is better not to accept the cargo on the vessel until a proper survey is carried out.