as shown in the picture, once a transaction is sent to the network waiting for confirmation, this "of 12 confirmations" appears, what does this "12 confirmations" really mean?
The number of confirmations is the number of new blocks built over the block that your transaction was inserted into.
If your transaction was inserted into block 1234567, waiting for 12 confirmations would mean waiting until block 1234578 is produced, AND your transaction is still in block 1234567.
This is to ensure that your transaction was not lost due a fork in the blockchain, or that your transaction was included in an uncle (orphaned) block.
As an example using the main Ethereum blockchain, the following image shows a transaction included in block 1436475. This image was taken when the highest block number was 1436476. This transaction has been confirmed in 2 blocks 1436475 and 1436476.