What is this part and what does it do?

rcknrbn wrote:
Noticed some damp folded towels in the small cabinet under the sink in the bedroom and assumed I had a leak from the p-trap or something sink related. Turns out this (pictured) device has an occasional drip. I assume it's a vacuum breaker or something similar. Is it repairable? If not, where do you purchase one? What purpose does it serve? Just something I haven't seen listed on some of the RV parts sites.

Thanks in advance.