help about opcode

How about telling us which processor you are talking about? Also you 
could most likely find a description of the opcodes of your processor on 
the vendor website.

http://www.ece.wisc.edu/~morrow/ECE353/arm7tdmi_instruction_set_reference.pdf

1AFFFFF9 means

1 = Condition: not equal
A = Branch (Link = 0)
FFFFF9 = offset

Branch if not equal
  PC := PC + (offset << 2);

This branch can initiate a jump in negative direction to an address that 
is 7 instructions or 28 bytes ahead.

The offset is given in two's complement to encode positive and negative 
numbers
http://en.wikipedia.org/wiki/Two%27s_complement

This is because it is a relative jump that can jump forward and 
backwards. So the offset is the number of instructions to jump (from the 
current position of the program counter), which can be positive to jump 
in forward direction or negative to jump in reverse direction.

An offset of FFFFF9 (which is -7 in two's complement) means, that a jump 
from the current position to a position that is 7 instructions earlier 
is performed.