When an xtor campaign tracker is present in a URL pointing towards your site, the information contained within is used to fill two parameters in the hit.
The xto parameter gets the full value of the xtor in the URL to indicate that the campaign it represents is the source of the visit. This parameter is only sent in the very first hit of any visit.
Once it is sent, the value is saved in a cookie deposited on the visitor's browser.
That's where the xtor parameter comes in. It represents the persistent campaign, i.e. a previous campaign that a visitor encountered that got saved in the above cookie.
This parameter allows for better attribution data, mainly in cases where a visit from an organic source resulted in a conversion, but the visitor had previously visited the site from a marketing campaign. That conversion will thus be counted as a conversion (prior attribution) for the campaign.