This is part of the Mosmatic nozzle assembly. This is an 1/8" nozzle that screws into the female outlet of the nozzle assembly. You would use this to change the degree or orifice size. Currently this comes in 25 Degree and 40 Degree with a variation of orifice sizes.
The orifice is the size of the hole that dictates how much pressure you get. It is dependent on how much volume (GPM/Flow) your pressure washer outputs. I would recommend the 40 degree nozzle for washing cars.