Meaning of Ryo(りょ) or Ri(り)
It means “I got it!”
It’s an abbreviation for Ryoukai(了解：りょうかい).
It’s a slang. Mainly young people use this word on LINE. (LINE is the most popular messaging app in Japan.)
On the messaging app, people prefer short word.
At first, young people use “Ryo(りょ)”, but nowadays they use “Ri(り)”.
Example Conversation[On LINE app]
A: Then let’s meet up at Shibuya at 1p.m.