Mastering a ship, or a mastered ship is when every researchable item for a particular ship has been researched and the highest item/part purchased for that ship. Significantly more credits and exp is rewarded for mastered ships. Note that every ship or "Liasion"/"Next Ship" must also be researched for the ship to be mastered.

 I was grinding through the IJN DD line to have access to multiple ships. I.E Sub, Kuma/Kita, and Fuso. I had the nessary ship exp and credits to research and purchase the Kuma, however decided to try and master the Assahio Type D. I researched all the items and purchased the highest all of the items only to find out that you also have to research each of the next ships for the ship to become mastered.... So now 10hrs of grinding thrown away to learn a simple NF2 lesson. Now im passing it on to you ALL!!!

/rant

 More to come on ship mastering as i figure it out...