After connecting Core to your bank accounts and downloading the transactions, you can match the transactions in both. Find Match pulls all the transactions which exists in Core and lets you match them with the imported transactions. If you do not find any matching transaction in Core against any imported transaction, you can create a new transaction. Please note that you can add a new transaction in Core that appears in your bank feeds, but not vice versa. Check the Core Help for details.
Note: Cloud Feeds does not import transactions that are older than three months.