The Roll Xero integration works in two areas. Firstly, it allows for the synchronisation of "Company" data between your Roll and Xero. Secondly, it allows the creation of invoices directly within Roll which are sent through to Xero and the confirmation from Xero back to Roll on if those invoices have been paid or not.
To link your Roll account to your Xero account, go to the setting tab of Roll (spanner at the top right of the application). Here, at the click of a button, you will be able to connect your Roll and Xero accounts. Please note to enable the integration, you must also be logged into Xero using your Xero master account details.