What is a Confirmation?
The process by which a Bitcoin transaction is verified and added to the blockchain. Each new block that's added after your transaction counts as an additional confirmation. Most exchanges and services consider a transaction secure after 6 confirmations (about 60 minutes).
Why It Matters
Confirmations measure the security of a Bitcoin transaction. When you first send bitcoin, the transaction enters the mempool as unconfirmed. Until it's included in a block, it's not final. A powerful attacker could still double-spend the same coins. Unlikely, but possible.
Each new block after your transaction is one more confirmation. More confirmations means more computational work an attacker would need to redo to reverse your transaction. After six confirmations, that work exceeds reasonable attack costs.
Understanding confirmations tells you when you can trust a payment has completed. Small amounts or trusted parties: one or two confirmations might be enough. Significant transactions: six or more is the standard.
How It Works
Your transaction gets one confirmation when miners include it in a block. Next block: two. Each new block adds one more. The longer the chain that follows your transaction, the more computationally expensive it becomes to rewrite that history. To reverse a six-confirmation transaction, an attacker would need to redo the mining work for all six blocks plus extend their chain further than the rest of the honest network. The six-confirmation standard arose because it represents about 60 minutes of mining time, a threshold that balances practical finality against the wait that most users find tolerable.