What I think that this means is that Costco can't go to a distributor in Vietnam, Cambodia, etc. where Omega watches are sold much cheaper than in the US, buy them, import them and then sell them at the store in the US.
Basically Costco is using the grey market to sell products cheaper. Omega wanted to block that, and to have Costco use an authorized Omega importer, that charges a whole lot more for the same item, because it is coming to the US.
This happens with drugs and other products all the time. The suggested retail price in the US is much higher than for the same item in a less affluent country. Omega wants to ensure that people in affluent countries pay more for items because they can.