A sacrificial barrier is something you put over your paintwork to be sacrificed instead of the paint. Technically, all coatings you put on your car are sacrificial barriers, as none last forever.
If your car's paintwork is bare, then the elements (UV radiation, rain, salt, pollution, ozone etc) will attack and damage it. To prevent this, you cover it in a sacrificial layer, which will absorb this punishment instead.
The oldest type of sacrificial barrier is wax, which is far less durable than your car's paint and needs to be replenished regularly. However, it is far easier and cheaper to top up your wax than to repaint your car.
As technology has evolved, we now have products that last much longer and are more durable than wax, to the point that some are more durable than the paintwork. They are, however, still technically sacrificial as they will wear down eventually but can be topped up or replaced.