It means that if you have two of the right sorts
of subproofs, then you are entitled to claim a biconditional.
The right sort?
Yes! One starting with the left side of the
biconditional and ending with its right side. The other starting
with the right side and ending with the left side.
It's not as hard as it sounds.
They
don't have to occur in the displayed order, but you must have each
one.
