Principle B is a statement in the government and binding theory of syntax that applies to pronoun
s. It has the opposite statement from Principle A
*Maryisaw heri in the mirror
*John likes Maryi's picture of heri
Maryi's father likes heri
The book about Maryi fell on heri
Maryi thinks the boy saw heri
Maryi saw herj in the mirror
Each of these examples corresponds to a tree drawn in Principle A, wherein the words are all exactly the new examples except that the Principle A ones have "herself" where the new one has "her".
Compare the pattern of grammaticallity judgments for these new examples with those for the Principle A examples.
Principle B is the inverse of Principle A:
A: An anaphor must be bound in its own clause
B: A pronoun must be free in its own clause
See Principle A for important context.