I don't think it is immediately clear at all what it means or why it is good to have a "rain interpreter".
The EVM already has general purpose set of opcodes for smart contracts, why do we want to reinvent that wheel?
How is a Rain interpreter different to the myriad of languages like solidity, vyper, etc. that are high level, battle tested and compile to the EVM?
How can this possibly be gas efficient, useful, secure, etc.?
Everything that Rain can possibly do could be coded in Solidity (the Rain interpreter itself is Solidity) so what's the point?