Maharbal was Hannibal's cavalry-leader during the Second Punic War. Many times he was important in Carthage's successes over Rome. In his Italian campaign, Hannibal always was able to have more cavalry than Rome, and so relied on them and Maharbal to give himself a big advantage.
Maharbal is most famous for what he said to Hannibal after the Battle of Cannae. According to Livy, a Roman historian, the conversation went like this after Maharbal said he wanted to go with the army to Rome immediately:
"I commend your zeal", Hannibal said to Maharbal; "but I need time to weigh the plan that you propose." "Assuredly", Maharbal replied, "no one man has been blessed with all God's gifts. You know, Hannibal, how to gain a victory; you do not know how to use it." -Livy, The History of Rome 22.51
In Simple English: "I like your enthusiasm", Hannibal said to Maharbal; "but I need time to think about what you want to do." Maharbal said back, "It is true that no one is perfect. You know how to win, Hannibal; you do not know what to do after."
Livy wrote this in Latin, so Maharbal's reply is often quoted in Latin, and is a famous phrase: "Vincere scis, Hannibal; victoria uti nescis."