Branch address displacement is used by branch instructions to go back to some previous instruction after execution of current instruction.It uses PC - relative addressing mode.
Say we have following sets of instructions :
1000 LOAD A,50
1004 MOV C , 8
LOOP : 1008 ADD B
1012 DEC C
1016 JNZ LOOP
So here PC value for 1st instruction is 1004 , for 2nd one it is 1008 and so on..Now if the branch condition which is based on value of C is not zero , it branches back to label "LOOP" which has instruction 'ADD" having address 1008.
Hence PC should be updated as 1008 instead 1020 which would have been the next address otherwise.
Hence PC = PC + displacement
==> 1008 = 1020 + displacement
==> displacement = -12
This way u may understand the notion of displacement in ur example as well.