Is 12 common words of key phrase is secure or not

I have a question about how secure that rvn core wallet key phrase(any other wallets).what if a person uses a brute force attack or randomly entering 12 key phrase and find someones 12 key phrase randomly or I was thinking wrong about this. And I was thinking that writing your key phrase in paper is called cold or any other way to called cold storage.IDK exactly what is cold storage