Basically its a policy that freezes the chain in carbonite after a certain block depth, so a 51% attacker can only re-org the chain so far before nodes just reject changes to blocks deeper than the limit. It doesn't prevent a 51% attack but makes it so they cannot do a deep re-org that could cause nodes to choke for hours and hours.

ABC currently does this with a rolling checkpoint of 10 blocks, so the worst an attacker can do is re-org from 10 blocks ago. This was however a hip-fired solution for the November attack pending a better system as cryptos4pz notes.

Small re-orgs of 1 or two blocks can happen naturally, but more than that is almost always the result of a malicious miner.