Kill hit percentage is just the measure how many bullets hit their mark. In Fighter Mode, only hits vs. enemy players do count, hitting structures counts as "miss".
In Attack Mode, both hits vs structures as well as planes & GVs do count.
And it's only as important as you take it

Looking at another players hit% can give you a general impression of his marksmanship skill, but not much more. Hit% depends on a lot of factors, favored ride, playing/fighting style etc. My own hit % varies by up to 50% from tour to tour depending on what planes I'm flying.
Kill points are the total points of damage inflicted on your enemies plus 1 additional point per kill. Shooting bombers gets you a lot of kill points because there's a lot to damage.