Dripping sound from valve near main inlet

[Roortic]

This valve is leaking it is connected between the main inlet the house line and a floor drain what is this valve?

 

[frodo]

looks like a trap primer, it is designed to leak
it keeps water in your p trap, p traps that are not often used , the water evaporates and the sewer gas stinks up the house
a trap primer prevents the evaporation

 

[Roortic]

It was spraying a steady stream of water i got a new one and replaced it now itleaks much slower. Problem solved. Thanks