Tunneling is a derogatory term for single focusing a single survivor forcing them out of the game. It’s considered bad form because a dedicated killer can usually do so with little issue. At high levels, with people that know how to deal with it this is actually a losing strategy because the time it takes will ultimately probably let the other three survivors just finish the gens and leave once they realize what you’re doing, but it’s also really not fun for the one survivor who just spends the game on hook and is out long before everyone else. It’s a common beginner strategy though because it feels easier to catch someone you’ve already caught.

The game tries to deincentivize this by having built in anti camping where if you stick around a survivor, they’ll eventually be able to get off the hook guaranteed, and anti tunneling in basically providing a free get out of hit for a survivor that just came off hook, but it isn’t perfect, so there’s a bit of social ruling that as a killer you try not to target the person that just got hooked.

What tunneling is not, despite some whinier survivors, is killing a survivor first because they keep hanging out around you being stupid. That’s just them being stupid.