Short passage to be explained like am 5

Guys, I usually get english but this time its all going over my head.

I know what finality means, I know both finalisation and justification upgrades. But the following did not make sense to me.

“finality requires a two-thirds agreement that a block is canonical, an attacker cannot possibly create an alternative finalized chain without:

• Owning or manipulating two-thirds of the total staked ether.

• Destroying at least one-third of the total staked ether.

The first condition arises because two-thirds of the staked ether is required to finalize a chain.

The second condition arises because if two-thirds of the total stake has voted in favor of both forks, then one-third must have voted on both. Double-voting is a slashing condition that would be maximally punished, and one-third of the total stake would be destroyed.”

What I understood:

• 66.66% of total eth staked needs to be vote for upgradation of the concerned block.

The above alone and not even the reasoning or how this voting happens.(like if they ask for finalising/justifying x block or is all of that done by only asking which chain is the correct latest blockchain state or what exactly is asked)

Can someone please simplify this for me?

I do not understand anything from the second condition paragraph, please explain me that paragraph in simple manner.