So checkpoints are reference points in the blockchain which the blockchain can be rolled back to in the event of a catastrophic attack. Basically until last night we didn't have any so if someone had wanted to kill Carboncoin they could have 51%ed us and orphaned every block back to block number two. Now we have a checkpoint in at block 60k so that is as far back as we would have to go if we got security tested - so things just got A LOT more secure.