Confirmations are when your Bitcoin transaction is included in a block, cementing its place in the blockchain and making it irreversible.
As this transaction from the exchange to your Bitcoin address is broadcast across the network, nodes will verify that the Bitcoin you wish to withdraw is actually available in the exchange’s Bitcoin wallet and hasn’t been double spent.
Once this is verified, the transaction will be added to the next batch/block of transactions and miners will then process this block. This processing of the block is known as a confirmation.
Every 10 minutes, new batches of transactions are processed and added, which will include all previous transaction history of the Bitcoin blockchain. So every 10 minutes, an additional confirmation is added.
A general rule of thumb is that 6 confirmations is enough to make the transaction permanent but sometimes, even just 2 confirmations is enough.