That means they have green mana symbols somewhere on the card, whether in the casting cost, rules text, or both, but they contain no other colored mana symbols
"Well you still have to pay the mana cost, and it's instant spells only, how bad can it be?" It means you can draw lots of cards with Brainstorm, blink Chun-Li to get her out of harm's way with an Ephemerate, or make your board untouchable with a Teferi's Protection every turn
While control decks typically leave a lot of mana on the board outside their own turn, this will raise alarm bells for your opponent, who may look to counter if possible
Return target to its owners deck only when game is over. Its worth about $6
The same applies to Merlin Rabbit who allows you to draw a card when he enters and leaves play
Varolz feel like he struggles to impress more than the many other three mana options