Confirmation is a state where a process or transaction has been encoded or written onto the immutable ledger called the blockchain. Confirmation first requires that miners validate a transaction by including it in the next or current block. But inclusion of the transaction in a block does not necessarily mean it is confirmed because the block has to be a certain “depth” or buried beneath enough blocks for it to be considered confirmed.
Transactions in Bitcoin are usually considered confirmed if they are at least 6 blocks deep, this means that at an average blocktime of 10 minutes per block, a transaction is considered confirmed in 60 minutes or an hour. Other services like exchanges may have faster confirmation times if they reduce the number of confirmations required before a transaction is considered spendable.
Think of confirmation as a document that requires multiple signatures for validation, where the more signatures that sign the document the more secure it is.