can anybody can tell me what is the difference between
relative and absolute addressing?
The syntactic difference is: A1 is relative, $A$1 is absolute. There
are other variations of addressing, namely: $A1 and A$1.
The effective difference arises when you copy and paste (or drag) a
cell. An absolute address (or part of the address) will not change.
A relative address (or part of the address) is modified in a manner
that Excel thinks makes sense.
For example, if A1 contains =B1+C1, and you copy and paste the into
A2, A3, etc, the formula in A2 will become =B2+C2. But if you A1
contains =B1+$C$1, then A2 will become =B2+$C$1.
Of course, whether or not that is desirable depends on the design of
your worksheet and what your intent is.
HTH.