This feature can be used on recurring bills and auto-reconcile rent payments from tenants who regularly use their own references when paying rent.
How to create a new bank rule:
In the "unreconciled transactions" list, go to the transaction you'd like to save as a rule and click on it.
Choose the category and then tick the box next to "Save the rule". Once you're happy, press "Save".
From now on, all transactions matching the description of the original transaction will be automatically reconciled.
Here's an article on how to change existing reconciliation rules