It doesn’t sound like "rido" — it sounds like "righto", because that’s what it is. Per the paywalled OED entry:
A. int. colloquial. Expressing acknowledgement, assent, or compliance; ‘OK!’, ‘that's fine’, ‘agreed’. Cf. righty-ho int.
It means exactly the same thing Right! means. Their earliest citation is from ...
What’s cooking, good looking?
Kia Ora (New Zealand)
G'day mate (Australia)
Whazzzuuuuup (yes you have to drag it out)
What’s with the face? (if someone looks angry or sad)
Greetings earthling (funny/nerdy)
Note that "What's doing" (suggested by @Lambie) is NOT grammatical.