A more thorough explanation:

A writer of the tallies is an officer of the Exchequer in English law. Their job is to write on the tallies the letters of tellers' bills. A tally is a stick cut into two parts and marked with notches to show what was due between a debtor and creditor.

For example, if someone owed another person money, they would use a tally to keep track of how much they owed. The stick would be marked with notches to represent the amount received. After the notches had been cut, the stick was split lengthwise into two unequal pieces. The longer piece, which contained a stump or handle and was called the ‘stock,’ was given to the person making the payment, and the shorter, a flat strip called the ‘foil,’ to the other party. If the sum involved was disputed, the two pieces could be fitted one to the other to see if they would ‘tally.’

The terminology has left a permanent imprint on our language. If you lent money to the Bank of England, tallies were cut for the amount: the Bank kept the foil and you received the stock; you thus held ‘Bank Stock’ of the amount recorded upon it. When the form of cheque was adopted, it was not indeed called a foil, but the part retained by the payer is still the counterfoil; and the word ‘cheque’ itself goes back ultimately to the same root as ‘exchequer.’

From early times tallies were used in the Exchequer and this lasted until 1826. The burning of a large quantity of old tallies led to the burning down of the old Houses of Parliament.

Overall, a writer of the tallies is an important officer in English law who helps keep track of debts and payments using a tally system.