Nothing, really. It's just a stock phrase. Japan has a few of them. It CAN be translated, literally, as "it is so".
Nano (ใชใฎ) is a "sentence-ender", something that people use to give their words more life when talking. Generally, nano is used by females to beautify their sentences.
Some ships that use nano include Haruna and Iku (I-19).
Desu (ใงใ) is a more formal sentence-ender that can be used by anyone. Typically, statements will end with desu, although it is usually not needed and people will use other sentence-enders to fit the situation, such as a verb-masu.
hi there, where are we going to go nano desu
57๐ 5๐
Japanese for "that is so"
used by Higurashi No Naku Koro Ni character, Rika Furude.
when said, it makes the speaker sound cute and adorable.
159๐ 43๐