This is a clearance rule, specified as 10 thou (the default rule).
On layer I3 in the design, you have a pad overlapping a track or another pad by 6 thou (hence the -6 in the Actual clearance column). The pad in question is highlighted.
The rule highlights errors from different nets.
In this case, you appear to have connected the pad highlighted to a track to pin 7, but the ratsnest shows this should be connected to pin 4.