This shows a plan view of a typical four-wheel forklift truck. Notice the triangle shown in red. This triangle is measured from the centre of the rear axle to the centre of the front wheel tyres. The reason that the centre of the rear axle is used is because the axle is only fastened to the chassis of the truck in the centre and is allowed to articulate in the case of four-wheel trucks.
The idea is to keep the centre of gravity, which is shown in blue, within the confines of the stability triangle. Of course as an operator it is not possible to see what's happening with this stability triangle and so instead it is necessary to learn of the conditions that may cause the centre of gravity to fall outside of this triangle.