The Young White Dragon (Monster Vault page 67) has the following trait:
On an initiative of 10 + the dragon's initiative check, the dragon can move up to its speed as a free action. The dragon can move through enemies' spaces and gains resist 5 to all damage during the move. Each time the dragon enters an enemy's space for the first time during the move, it can use claw against that enemy. If the attack hits, the target also falls prone. If the dragon cannot use a free action to take this move due to a dominating or stunning effect, then the effect ends instead of the dragon making this move.
How should I play it? I don't understand what "The dragon can move on an initiative of 10 + the dragon's initiative check" means.