National  treatment:

It is the principle of non-discrimination as between an imported product and a like domestic product. A Member is prohibited from providing less favourable treatment to  an  imported product than provided  to a like domestic product (Article  I11 of GATT 1994).  

An  exception to the national treatment is provision  of  subsidy  for  the production of domestic products under certain disciplines without providing such subsidy to the like imported product.

You  can easily notice that these two basic principles are meant to ensure that there is unhindered competition ih trade. MFN principle ensures competition as between goods coming  from  different countries. National  treatment principle ensures competition as between an imported product and a domestic product.