Principle B is a statement in the government and binding theory of syntax that applies to referring expressions -- that is, pretty much any noun-phrase that is
not an anaphor or pronoun (which are covered by principles
A and
B respectively).
Examples:
*Shei saw the womani
S
/ \
NP VP
/ / \
N V S
she said / \
NP VP
/ / \
N V S
Bill thinks / \
NP VP
/ / \
N V NP
George saluted \
N
Mary
*She
i said Bill thinks George saluted Mary
i
A referring expression must be free.
See Principle A for important context.