Legacy Tracking refers to a past versions of ConversionRuler which originally tracked using the Tracking Parameters:
An example of how this looked was
http://www.example.com/?c1=ppc&source=goto&kw=cheap+dog+food
Due to the large number of sites which also use source as a Tracking Parameter, we switched our default tracking to use crsource instead, and make this an option in your Site Settings.
In general, we recommend turning Legacy Tracking off. While we track more traffic for you, it means you'll have a lot more garbage data in your reports.
Turning Legacy Tracking off with the following steps: