I guess I should've explained a bit more.
When used as an adverb then there's not really a difference between yno and yna:
Mae dyn yno/yna "There's a man there"
In addition to that, yna is used in the contruction y ... yna meaning "that ...":
Wyt ti'n nabod y dyn yna? "Do you know that man?"
Yna is often shortened to 'na in the colloquial language:
Mae dyn 'na
Wyt ti'n nabod y dyn 'na?
Yno can also be shortened to 'no but it's much less common and probably belongs to very specific dialects, as opposed to 'na which is common all over Wales. You might never come across 'no but by all means use 'na.