It is possible that bodyguard (literally: guardian of the head) was a position at court.1 In this case, Achish would reward David with status and position if he fought well in the battle. Alternatively, Achish was calling on David to fight in his presence, so that he could keep an eye on him.
2 David said to Achish, “Very well, you shall know what your servant can do.” And Achish said to David, “Very well, I will make you my bodyguard for life.”