Answer
A contra entry records a transaction affecting both the cash and bank columns of the same cash book—typically paying cash into the bank or drawing cash from the bank. Both sides of the entry appear in the cash book (one column debited, the other credited), marked with a 'C' to show no other ledger posting is needed.
Pay 300 cash into bank:
Cr Cash column 300 (money out of till)
Dr Bank column 300 (money into bank) -- marked CReal-world example Banking the day's takings is a contra entry moving money from the cash column to the bank column.