What was the significance of the merrimack versus monitor battle?

Answer 1

Tactically the battle outcome was indecisive.

Strategically it was a Union decisive success, because the Confederate ironclad never more attempted to break through the blockade and engage the Union Navy, thus remaining confined within the artillery range of Charleston's forts and becoming therefore a "warship in being".

From the technological point of view it was the first fighting between ironclads.

Furthermore it marked a superiority although not decisive of the iron armours against the smooth-bore guns.