My understanding is that NIP-13 defines a mechanism for updating an event repeatedly event with a special tag value, before it is published, until the hashed event id begins with a certain number of 0’s
The idea being that the publisher needed to apply a certain level of effort in order to generate the event (and therefore is prevented from spamming by the consequent economics).
Then relays and clients can filter messages based on the level of difficulty the events passed to be created.
Hope this makes sense. I’m pretty new here, so this is just my (probably naive) reading of NIP-13