What does it mean that “Scripture interprets Scripture”?

In sensus plenior it means that all the answers to the riddles are found within scripture.

Jesus says we should let our light shine before men, yet we should not let our left hand know what our right hand is doing. There are at least twelve different interpretations of this "apparent contradiction" by people using the literal-historic methods.

Jesus says Goats are on the left and sheep are on the right. So we should do the works of God for which we were created, but we should not let our 'goat nature', the flesh, interfere with the spiritual works we do. We should not be claiming credit for God's work as Nebuchadnezzar did.

So the Ninevites who did not know the left from the right, did not know the flesh from the spirit.

The sword-word (in Eglun and Ehud) was pulled from the right (spiritual) thigh-will with the left-flesh hand-works. God's spiritual will was accomplished by men working in the flesh.