In an assembly instruction operations are performed on operands. Operands have in MachineCode the format of binary numbers.


For ease of use operators and operands are often replaced with mnemonics (symbolic names  -> AssemblyMnemonic ). 
To asign a symbolic name to either an operator or operand a pseudo instruction is used.

OPERATOR     OPERAND

becomes:

OPERAND      =            SYMBOLIC_NAME
             OPERATOR     SYMBOLIC_NAME

thus:

ADD     01000100

becomes:

NN1     =       01000100
        ADD     NN1

In the first line NN1 = 01000100
the symbolic name NN1 is called a label , though when the symbolic name NN1 is used in the second line ADD NN1 it is called a symbol.