Ordinals are an ordered numbering system for satoshis. It is a metadata that exists outside of Bitcoin from a set of rules that give each Satoshi a serial number and then they ascribe meaning to them. Like the first Satoshi mined after the halving is "rare".

It's 100% absolute bullshit. It's a way to reduce fungibility of Bitcoin and IMO it's an attack. Not that I think anything should be done about it except ignore it.