Meld

Meld is a keyword action introduced in Eldritch Moon and means to turn a Double-faced card upside down, if you control a second card it can meld with.

Rules

701.37. Meld
- 701.37a Meld is a keyword action that appears in an ability on one card in a meld pair. To meld the two cards in a meld pair, put them onto the battlefield with their back faces up and combined. The resulting permanent is a single object represented by two cards. See rule 712, “Double-Faced Cards.”
- 701.37b Only two cards belonging to the same meld pair can be melded. Tokens, cards that aren’t meld cards, or meld cards that don’t form a meld pair can’t be melded.
- 701.37c If an effect instructs a player to meld objects that can’t be melded, they stay in their current zone.
  Example: A player owns and controls Midnight Scavengers and a token that’s a copy of Graf Rats. At the beginning of combat, both are exiled but can’t be melded. Midnight Scavengers remains exiled and the exiled token ceases to exist.

From the glossary of the Comprehensive Rules (June 7, 2024—Modern Horizons 3)

Meld: To turn two members of a meld pair so their back faces are up and combined into one oversized Magic card. See rule 701.37, “Meld.”

"Double-faced" always refers to the physical card. It is neither a characteristic nor a copiable value. For example, a token can never be a double-faced permanent even if it's a copy of one.
Each face of a double-faced card has its own name, types, subtypes, power and toughness, abilities, and so on. While a double-faced card is not on the battlefield, consider only the characteristics of its front face. While a double-faced card is on the battlefield, consider only the characteristics of the face that's currently up. The other set of characteristics is ignored expect for the converted mana cost. The back face has by definition the same converted mana cost as the front face.^[1]
A double-faced card enters the battlefield with the front face up.
The back face of a double-faced card can't be cast.