[2020] Noob questions

Just started playing a couple days ago from a YouTube ad and it's really refreshing game. I honestly love the universe and 2 region deck building style.

What does power mean for the card that can't be blocked by less than 3 power? Is that the attack, health or mana cost? Kinda unclear.

Also there is a spell that damages all attacking minions with a attacking minion. After you wipe a board some attackers just return and don't attack even after their blocker is gone,? Why is that? Is there some easy wiki page to read faq etc?

I wish you could zoom in by right clicking cards when you buy em.