The easiest way to understand Caster is to look at a bike (when viewed sideways on). 
If you draw a line down from the centre point on the handlebars down the forks to the ground (on a car this would be the suspension strut), this indicates the ‘pivot point’.

Then draw a second line vertical through the centre of the wheel and where the line touches the ground this shows the ‘tyre contact point’.

The top angle that has been created is what is known as the ‘Caster Angle’.

If this top angle is reduced then stability is less, therefore it is important to get the Caster set to its optimum angle for the best possible control.