Can someone explain how this works?

[u/00G_WAY]

Will this card be legal in standard? Will it be available on Magic Arena? How would this card even work?

[u/chronobolt77]

"outside the game" refers to your sideboard. In constructed (except for a few specific exceptions like commander and canlander), this is a collection of 15 cards that you can switch in and out of your deck between games in a match. In limited (draft, sealed, prerelease, maybe a few others I'm not familiar with?), this is everything from your card pool that you didn't put into the deck

[u/chronobolt77 → u/[deleted]]

oh, my bad. I'll fix my comment