A transaction is confirmed when it is included in a block. Every subsequent block constitutes another confirmation of the transaction. Often you will see waiting times talked about in terms of confirmations: “please wait until your BTC has received 3 confirmations”, “STRAT will be spendable after 16 confirmations”, etc. This is because the more confirmations a transaction has, the harder it becomes to roll back the blockchain and undo the transaction. After a few confirmations it becomes impossible (in any practical terms) to undo a transaction.