Your soul mate in a non-romantic sense. You cannot live without this person; they complete you. They balance you. You would be lost forever without them. Moirails are not just "best friends". It's so much more than that. Moirail is the ultimate bond that you can feel with someone.
Equius and Nepeta are moirails. She's bubbly and cute and happy, so she can defuse him when he flies into a rage, which is quite often. He's strong and protects her. They couldn't live without each other.