MoneroPulse is a DNS based system which regularly checks for a list of blockchain checkpoints stored in the DNS TXT record for a few monero domain names. This system allows emergency checkpoints to be set to mitigate an attack or accidental fork in the chain.
By default, this check is advisory (ie, you will get warnings if you are not on the chain that these records hold). The intent is to leave the choice to node administrators which chain they consider "the" chain, since those servers are under the core team's control. You can elect to have those checkpoints be authoritative by running bitmonerod with
--enforce-dns-checkpoining instead. This is encouraged for nodes running unattended.
Several servers are used, and the code checks for both the chain of trust (if available via DNSSEC) and for differences in the records supplied by the DNS servers, to give some degree of resilience against attacks and network reliability issues.