That's where the blowby gasses from the engine are expelled. Huwag mong isasara at maghahanap ng ibang lalabasan yung blowby gas. Baka sa valve cover gasket lumabas. Since may kasamang traces ng oil that dirty up the intake manifold, some people just disconnect the hose and install a breather filter pero pag na saturate na yung filter at hindi mo nilinisan yung engine bay naman ang dudumi. Some install an oil catch can along the breather hose to separate the oil from the gas para malinis na air pa rin ang napupunta sa intake.

HTH