Dream was added in Beta 51.1, obtainable through the new Artifact Ambition, a replacement for Will.
Dream functions pretty much like a Mulligan, allowing you to choose a card of your choice in your hand, return it to your deck and draw 2 new cards.
Since Ambition adds this to the top of your deck at the end of every third turn, it will be the next card you draw. You can then use it whenever you want to renew a bit of your hand.
From the moment you draw this to after you've drawn your new cards, you will have the same amount of cards in your hand (as you use this and return 1 to draw 2), so you don't have to worry about milling yourself.
Despite what the card text implies, you are not required to have one other card in hand in order to use it. Dream can be used even when it is the last card in your hand, allowing you to draw two new cards for free.