When a new block of information has been verified for addition onto the blockchain, it has one confirmation. Each additional blocks added onto the blockchain gives the referenced block another additional confirmation.
The further back a block is on the blockchain, the more costly it becomes to reverse it (through malicious means). Block confirmation is typically used by crypto exchanges to consider a cryptocurrency transaction final to protect themselves against bad actors who aim to reverse transactions after depositing (known as double spending).
gm is the short form writing for good morning. Used extensively within the crypto community where individuals greet each other casually. On the other hand, good night (gn) is used when people sign off for the day.