A banneret is a knight who inspires greatness in others by committing brave deeds in battle. The mere presence of one in a hamlet is enough to cause some orcs and bandits to seek easier prey. A lone banneret is a skilled warrior, but a banneret leading a band of allies can transform even the most poorly equipped militia into a ferocious war band.
A banneret prefers to lead through deeds, not words. As a banneret spearheads an attack, their actions can awaken reserves of courage and conviction in allies that they never suspected they had.

Source: Wizards of the Coast - Sword Coast Adventurer's Guide