Gas struts, gas springs or gas dampers are car parts which are generally sold in pairs. A boot gas strut (in this case) is fixed to the boot of the car on one side and to the car’s body on the other.
When the boot is opened, the gas struts extend by means of pressure inside. This is necessary to block them once the boot is open thereby preventing it to fall back down on its own. The force of gas struts is put in newtons. Without boot struts, the boot of the car wouldn’t be able to remain open by itself as it is a rather heavy part of your car.