Kitchen plumbing issues

[p3arljam]

Hopefully someone can help me i had both of my double sinks fill up with water so i took the trap apart it was clear and i then snaked the pipe going into the wall with an auger. After putting everything back in place i let the water run for about two minutes and the sink started to slow fill up again, I looked under the sink and the area marked with a blue arrow was dripping very slowly from right under the top piece. What is the piece called, what is it used for and whats my next step.

Thanks
Charlie

 

[Eman85]

I'm not a plumber but it looks like that might be a studor vent, or is supposed to be one. Does it unscrew?

 

[Sponsor]

 

[Breplum]

called an AAV (air admittance valve). supposed to be water tight but never really can be relied on.
You need your drain cleaned properly with a power snake, long enough to clear it.