what are function of this pipe?

[Ndelilio Nnko]

i dont know the function.of this pipe in my r4, i change water pump.but my r4 radiator still boil water and sometime it heat? can anyone help m out??

 

[Tripyrenees]

That is the bleed pipe coming from the highest point in the cooling system (top of heater matrix)

Run you engine and unscrew the end, air will come out and the very warm coolant. Should fix your issue.