What’s this pipe outlet looking thing on the wall below sink?
Most likely, that is a drain clean-out. If you unscrew the cover, you get a fairly straight shot to your main sewer line. This way, if you get a clog (often tree roots penetrating the pipe) in your sewer pipe leading to your municipal waste system, you can get tools (rotary cleaners, inspection cameras, etc.) into the line.

Just leave it be and make sure you can get at it if necessary.