Caster is a front suspension setup / adjustment measurement.
It "aids" a vehicle going straight down the road. And, it "aids" steering / wheel "return" when making a turn.
Caster is positive -- 2 degrees is the stock measurement / setting.
Negative caster will cause "steering problems" / "wandering".
You can read many posts here and on the bb about the effects of negative caster when people do "lifts" / SOA.
Jeff has documented the changes he has made on some of the "builds" he has posted on this site.