Chasing Curiosity. Empathetic Accelerationist. Founding Engineer @icebreaker. YC S16, KB7. 
Icebreaker.xyz/tayyab

It's a degree-5 function from a size-192 list of primes mod 2^31-1 to a size-176 list of primes mod 2^31-1, which evaluates to true only if the last eight items of the input are the Poseidon2 hash output of the first sixteen items of the input. This is useful for making hyper-optimized STARK proofs of hashes, which in turn is useful for STARKing anything blockchain-related, and potentially a (quantum-safe) replacement for verkle trees.

duh ! XD

Although on a serious note I love that Vitalik actually answers these questions to at least give us a fighting chance of understanding.