A confirmation block is mined after the block containing a transaction. Each additional block increases confidence that the transaction’s block will remain on the winning chain rather than disappear in a fork. section 12.2.3
Why six confirmations?
Suppose a wine seller sees Alice’s 5 BTC transaction included at block height 10. Instead of accepting payment immediately, the seller waits for six more blocks. Wallets and exchanges commonly use this practice; it usually takes about an hour, though it does not provide absolute assurance against a later reorganization. section 12.2.3
Figure 12.6 A fork in the blockchain: two miners publish a valid block at height 3 (meaning 3 blocks after genesis). Later, another miner mines a block at height 4 that points to the second block at height 3. As the second fork is now longer, it is the valid fork that miners should continue to extend. Note that arrows coming out of a block point to the parent block (the block they extend).
Trace how a later block makes one competing fork longer, illustrating why blocks added after a transaction strengthen confidence in its chain. section 12.2.3
