M.2 What is a bank reconciliation and why should I review it? 

A bank reconciliation is a procedure for making sure that everything coming in and out of the bank account is recorded in the bookkeeping, and everything recorded in the bookkeeping is accounted for at the bank.

The bank reconciliation provides strong oversight for your bank account to ensure that all transactions coming in and out of the bank account are appropriate. Note, this especially includes fraud detection where unauthorized withdrawals may happen; the bank reconciliation is one key way of identifying these issues.