A master seal is an item in the Fire Emblem series. They used to upgrade a base class unit into an advanced class unit.
Master Seals change the class of a character between their sets of classes. Each character has three class sets that they can switch between, which, mostly, include one base class and two promoted classes. Whenever a character uses a master seal and they change their class they return to level one, and their stats will be modified based on the growth rates of their current class and their new class.
Characters that are in a base class can use a master seal once they reach level ten. They can then switch between the base and special classes in their class sets.
However, characters in promoted classes that are level ten or over can use a second seal to change between the base, special, and promoted classes in their class sets.