Here we go...
Ok, kills is the number of kills, as you deduced. The (5 Ancient) means that in 5 kills, they become an ancient ghost armor (or whatever name is given to the level). This is called leveling up, and the unit gains extra HP, sight, and stuff. It is good, as the unit becomes stronger. Kills are passed over when morphing.
Sight is the distance a unit can see. Notice how the fog of war (blackness in which prevents you from seeing the enemy) moves away a certain amount from units? This is the site. Units cannot see a unit unless it is within their sight range. If within the sight range, the unit can tell the unit is there, and may attack them if they are a foe. Note that just because you can see a unit on your screen doesn't mean the unit can see it. If the foe is within one of you unit's sight, only that unit (or others in the sight) can attack it, as it must be within the sight of any individual unit to be attacked by that unit, not just within one unit to be attacked by any.
Simple enough.