It is a quarter in the second hour = 1:15
Thinking like this helped me.
I think na means "NAch" (German) in the sense of "on the way to". So one 1/4 on the way to the second hour. That's my way to remember it.
Shouldn't it be
je čtvrt na jednu? Why is there dvě (2) ?
No. Please read the comment by Dorran6 and the answer by endless_sleeper to abasc.
"Je čtvrt na dvě." means it is one quarter of the hour leading to 2 o'clock.