Traps are an integral part of any waste system and are generally situated on the outlet of an appliance (to which the waste is attached). The purpose of a waste trap is to provide a water seal between the waste water pipework and the internal environment of the property, thus stopping any foul air entering the building which would be both hazardous and unpleasant.
 
 
 
40mm bath P-trap with a shallow 19mm seal in white.