What does Ryuta mean in japanese?

Luna is technically correct, however Ryuta (りゅた) is not a real name. I suspect you mean Ryuuta りゅうた (often double vowels are romanised as just a single vowel, even though the difference is actually quite important).

Ryuuta is a boy's name and can be written:

隆太 (noble or prosperous, and wide, broad or fat)

柳太 (Willow, and same as above)

竜太 or 龍太 (Dragon, and same as above) 

流太 (flow, and same as above)

太 (ta) is a common ending for boy's names. I guess it probably originated as something of a wish the for them to grow up big and strong (broad-shouldered and all that)?

琉太 (りゅた , Ryuta)

琉 means: precious stone, gem, lapis lazuli. 

太 means: plump, thick, big around.