MFN treatment:

It prohibits discrimination between one country  and another. A country that is a member of  the WTO cannot provide an especially beneficial treatment to any country in  the matter of export and  import. In  specific terms, this provision (Article I of the GATT 1994) says that when a country that is a member of the WTO  (now on called  simply "Member")  gives any benefit  in  the matter of export and import  to any country (whether Member or not), it must provide  the same benefit to all other Members immediately and unconditionally. The title is derived from the fact that any concession given by a Member to the most favoured country has also to be extended to all Members.

Examples:

1.  A Member cannot give lower tariff on a product to a country as a reward for receiving some aid or investment  from that country.  Such form  of reward will violate the MFN treatment principle.

2.  If a Member is annoyed with another Member for criticising it in the UN and  punishes  it  by  stopping import  from  that  Member country,  it  is violating the MFN treatment principle.

All tariffs  and  import rules have to be non-discriminatory as  between  the Members of the WTO.