Hello!
A biconditional statement is when one conditional statement and it's converse is true.
A conditional statement is an "If hypothesis, then conclusion" statement.
A converse statement is an "If conclusion, then hypothesis" statement.
Looking at this, both of these statements have their hypothesis and conclusion switched.
Thus, a biconditional statement is a "Hypothesis IFF conclusion." In this statement, IFF means if and only if.
Therefore, when IFF is used in a biconditional statement, it means if and only if.