Hello. I don't quite understand what the -O rule gives

[pdo]

sorry for the stupid questions, but I don't quite understand what the -O rule gives optimized-kernel-enable. If it takes too long to explain, give me a link, I can't find it

Optimized kernels give you faster performance in a trade-off with some kind of limitation, usually the length of the password candidates.  For example, the "pure" MD5 kernel will accept candidates up to 256 bytes in length, but the "optimized" kernel limits this to 31 bytes.