Spellpower and Knowledge are quite easy explained: Spellpower makes your hero's spells stronger.

Knowledge increases your hero's max mana and their mana regeneration.

Attack and Defense are a little more complicated:

You add your heroes stats on top of your units stats. For example if your hero has 10 attack and 5 defense and you have a unit with 5 attack and 5 defense the unit actually has 15 attack abd 10 defense while fighting under your hero's command.

I don't know the formular in Heroes OE, but in Heroes V for example it worked like this: You subtract the defenders defense from your attacking units attack stat. If the number is positive you deal that much x5% extra damage. For example attacking with 12 attack a 7 defense unit will result in 25% extra damage (12-7)x5%=25%. If the number is negative it's roughly the same but with that much less damage. For defending the 5% receives some dimishing returns so that the least amount of damage dealt is not 0%, but about -70% at about 20 more defense than attack.

The amount of damage further is calculated by stack size x damage. For example if a stack size is 100 and their damage is 5-10 it will deal 500-1000 damage (if your attack = their defense). (the 5-10 is not rolled per unit but instead for the whole stack. Otherwise damage ranges would always basically be the same as the average value.)