[Plumbing] This is leaking water out of a bathroom sink drain and down the wall. What is it and what will happen if I just cap it? And what would be the best way to cap it?

I mean, other places sounds like a good start, cause I don't think the drywall is loving the current setup. Is it broken and in need of fixing or was it a bad modification by the previous owner that needs to be replaced or can I cap it without worrying that I'll create a problem with drainage? These are the things I don't know that I was hoping reddit would be able to tell me, because other than removing a clog or two over the years, I've never had to deal with plumbing before.

Edit for additional info: we are looking at a 4 way junction between the sink drain, the sink overflow drain, the pipe to the P-trap and this thing that looks like it's supposed to be a vent of some sort but it gushes water as it drains from the sink every time the tap is run. Trying to figure out what the vent is and if I can close it without causing drainage problems.

Decided to take the whole thing apart and try to get a better look. Turns out it's not a vent, it's a break in the pipe corner. Going to replace it, should be good to go. Appreciate you taking the time, though, have a good one.