This would be a stat that a pet can do damage with. Strength, for melee pets (Ie: Earth, Water, Wind Proficiency), and Intelligence for a magic based pet (Ie: Light, Shadow, Fire).

A primary offensive stat should be referring to either intelligence or strength as Zelly said. The easiest way to remember which proficiency uses which stat is by looking at the different attacks they have. If the attack does magical damage, you use intellect which increases the magical damage. If the attack does physical damage, you use strength which increases the physical damage.

The main types of gear are:
Myrmidon - strength/dexterity - used by non-tanking physical damage pets
Evoker - intelligence/dexterity - used by non-tanking magical damage pets
Assassin - strength/agility - used by physical damage tanks
Illusionist - intelligence/agility - used by magical damage tanks

In the second dungeon, you can find mythic versions of these four types. The dexterity of myrmidon and evoker is better for non-tanking glass cannon pets where it helps increase their damage. Agility is better for tanks for the mitigation.

Btw, when using relics be careful to avoid relics with the one type of damage. If what proficiency you use calls for magical damage get a relic that only has magical damage and one that's only physical damage if it calls for that instead.

Thanks that explains alot of things!

Let me know if you need any more help!