Q: What is Caster?

Caster is the forward or rearward tilt angle of your steering axis. For example, on the Model 3 Performance Multi-link Suspension the steering axis is relative to the placement of the upper and lower ball joints and their offset relative to a vertical line. So if you were to draw a line between the upper and lower ball joints and measure the angle of that relative to a vertical line through your hub that would determine your caster.