According to the documentation i've found for NfDump, obyt is the number of output byte.
What does it means ? In all my data, obyt is equal to 0 . Does it means that all the packet are dropped ?
No, it most likely means that you're looking at uniflows -- NfDump is reporting both halves of the conversation separately (which is typical for NetFlow) and so the byte count you're looking for is either going to be in "ibyts" or just "byts". (BTW, there are corresponding counts for packets)
If you want to figure out how many bytes were passed in each direction, then, you'll need to match up the uniflow halves. So, let's say you're looking at a host (IP A) making a single web request to IP B. You should be seeing two entries in your dump: The first one would be Source Address IP A with Source Port in the ephemeral range (say, 45678) to Dest Address IP B and Dest Port 80, on protocol TCP, sending a handful of packets and bytes. The second one would be the exact reverse, Source Address B and Source Port 80 to Dest Address A and Dest Port 45678 on TCP, sending probably many more packets and bytes. You use the Source IP, Source Port, Dest IP, Dest Port, and Protocol to match up the two halves (Note also that they should also be similar in Start Time, but probably won't be exactly the same)