When a miner mines a valid block, call it block #231765, and broadcasts it to the blockchain, the mining for block #231766 starts right away. When block #231766 gets mined, all the transactions in block #231765 are now one block deep in the blockchain. This means that the network is essentially “confirming”, or, trusting, that all those transactions are valid. For a transaction to have “one confirmation”, it needs to be in a block that is 1 block deep on the blockchain.
As blocks get deeper and deeper down the blockchain, they become increasingly immutable. By the time a block gets 3-6 block confirmations, the contents of that block are virtually unchangeable. If one wanted to change or alter a transaction one block below the current block, theoretically they’d have to rehash and remine that block while also mining the current block, which is virtually impossible.
- Antonopoulos, Andreas M. Mastering Bitcoin: Unlocking Digital Crypto-Currencies. 2015. pg 29.
- 3 Things to know about Confirmations – buybitcoinworldwide.com