the offensive value is an easy tool to allow the player to compare its units. the offensive value describes the offensive capabilities of a unit in terms of damage per second and burst damage potential. the formulas are rather complex, but you can easily compare the offensive value of two ships to see which one deals more damage.
Avatar modification which add an additional weapon do usually also have an offensive value bonus. the additional weapon then uses this offensive value for its damage calculation (the old weapons remain untouched) while an offensive value bonus without and armament change increases the damage of the already installed weapons.

well the system value, i know is a set of values that determine both special energy recharge rate & also how quickly system repairs are made. not sure exactly how they do this but i do know the higher the value, the faster it is in both cases.

Defence value i imagine works exactly the same as the offence value only in terms of shield & hull points.

Perhaps it would be make a sticky with this information for the sake of those newer to the system as well.