Initiative - this is a special concept at every unit, with some initiatives comparing to each other and those whose is higher will go earlier, while those whose is lower will go later. Don't confuse it with speed - speed determines the number of hexes per turn your unit will travel, and initiative - who goes earlier. 

When you press the button to deal damage, the higher your initiative is, the earlier your unit will go in that turn of the same hex. 

Initiative can be influenced by spells - for example, the magic "Leading Strike" not only deals damage, but also reduces the enemy's initiative on the next few moves. That is, if you see that the enemy's unit is going earlier than yours, then you can use this magic and slow it down so that your unit goes first.