Oh let me explain.

Tldr: you can proof something happened before a certain moment in time by anchoring it to bitcoin. Nip03 is just putting those proofs in an event.

Nostr events have a time/publish date field, where it is written when the creation supposedly took place. But thats just a number, and the author is totally free to lie about that number, nothing is really preventing that.

But we do have something else, called 'Open Time Stamps' (OTS). It provides a method to proof that something is ATLEAST a particular age old; i.e. it proofs something existed at that moment in time, but could be older.

It does that by putting a cryptographic hash via a bitcoin transaction into the Bitcoin blockchain; given the nature of Bitcoin's proof of work chain, an outside observer can be very certain as to the approximate date/time associated with the block that particular transaction/hash resides in.

Now all that is in the blockchain is this simple hash; but that hash is the product of a process where a (potentially large, say millions) collection of files/things are condensed into said hash, and for each of those files/things there exist a proof that allows an outside observer to verify that that file/thing indeed was part of the creation of the hash; ergo the file/thing must have existed at the time the hash ended up in the blockchain. (Hope im still making sense here).

In short: its a method to tie countless of things to the bitcoin blockchain in a very cheap/efficient/scalable manner to say something verfiable about time about those things. I.e. you can't produce nostr events after the fact, pretending that they are old (well you can, but you can't and also provide such an OTS proof, because you are not powerfull enough to rewrite the blockchain).

NIP-03 is a simple standard where you put the timestamp proof of an event in a seperate event. This for example could help you defend yourself in situations where your keys are stolen; assuming you consistently timestamped all your stuff, and people expect such OTS proofs, the key-thief cant create legitemate events pretending to be statements made in the past.