What does it mean to "top deck"?

What does it mean to "top deck"? - Close-Up Shot of Scrabble Tiles on a White Surface

I see this term getting thrown around a lot in the Hearthstone community and not a lot of context is there to fully understand its' meaning.

What does it mean if someone is "topdecking"?

terminology hearthstone

Pictures about "What does it mean to "top deck"?"

What does it mean to "top deck"? - Close-Up Shot of Scrabble Tiles on a White Surface

What does it mean to "top deck"? - Close-Up Shot of Scrabble Tiles on a White Surface

What does it mean to "top deck"? - Close-Up Shot of Scrabble Tiles on a White Surface

What does top deck mean?

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