Basic transaction data (such as amount, currency, date, etc.) is automatically sent to Xero, once you setup the integration.
You can also export transaction metadata (attachments, notes, Xero Categories, etc.) to save you time. To do so, go into Report in your left-hand-side menu, click on Xero, select which transactions you want to export metadata for, and then press the Export to Xero button. They will then show as Pending until Xero accepts or declines the data. Transactions with metadata which have not been exported to Xero will remain in the To Export tab, and those which have successfully been exported to Xero will move to the Exported tab.