An enemy unit that has had a charge declared against it must hold or perform a charge reaction - shooting at the chargers or retreating from the threat. Once the charge reaction is complete, the charging player can declare a charge with another unit, until they have declared all the charges they wish to.
When you declare a charge, the target unit isn't necessarily going to sit there passively waiting for your warriors to bear down on it. Depending on the target's armament and general fighting ability, it might choose to steel itself for the charge's impact, all the better to fight off the attackers, fire a volley of arrows or other missiles into the oncoming chargers, or simply decide to opt for discretionary valour and flee from the foe.
The response your unit makes when a charge is declared is referred to as a charge reaction. As your opponent declares a charge, you can declare a charge reaction for your charged unit. There are two types of charge reaction: Stand and Shoot, and Flee! A unit that does not make a charge reaction is always considered to Hold.