Posidonius writes, that Fabius was called the buckler [or shield], Marcellus the sword of Rome. Certainly, Hannibal himself confessed that he feared Fabius as a schoolmaster, Marcellus as an adversary: the former, lest he should be hindered from doing mischief; the latter, lest he should receive harm himself.
tl;dr:
Fabius, the father of ‘Fabian Tactics,’ was known as the shield of Rome, because he stopped the damage that Hannibal was doing to the Roman Republic, and Marcellus was known as the Sword, because he was a capable commander who could face Hannibal as a serious adversary. Hannibal later confessed that he was afraid of Fabius, because he could stop him from doing damage, and he feared Marcellus, lest the guy turn a battle around.
Source:
Plutarch, John Dryden, and Arthur Hugh Clough. "Marcellus." Plutarch's Lives. New York: Modern Library, 2001. 414. Print.
Further Reading:
Your post, the last sentence is lacking its ending.