Ryuou is what the Wano folk call Haki.
Lol! This has already been discussed and cleared.
Ryuo is generally called armament haki or more specifically flowering Sakura. Just like all variations of haki (CoC, coo and CoA) are generally referred as haki,wano people refer all variations of armament as ryuo.
But as all variations of haki has different usage, so does ryuo because it builds on basic armament haki usage.
Basic armament haki allows user to hit Logia and coat one's body part. This is what we learnt prior to Wano.
Ryuo in wano for Luffy and Zoro is going beyond that basic armament haki that is to make blade black, create shock barrier or destroy objects from within. That's what Luffy and Zoro are training for and not basic armament haki