What do you mean by contra entry?

In the three column cash book there will be some cross or contra entries i.e., transfer of money from cash to bank (amount deposited) and vice versa (amount withdrawn from bank for office use). In all such cases both entries occur in the cash book and no ledger entry is required. This is indicated by a contra sign (C) in the folio column indicating thereby that the double entry aspect of this transaction is complete and it requires no posting to the ledger.

"Deposits in the bank of cash: debit bank column 'To Cash' and credit the cash column 'By bank'."

In this case there will be no posting to any other book of account and it is customary to indicate the fact by placing in the folio column of cash book against such entries C which stands for Latin word contra.