To put it simply, it is like a "melee attack" in paintball, a way to get someone out who is close or right next to you without having to fire. And no, it doesn't involve hitting, like you said, a tap on the shoulder or even calling them out if you got the drop on them.
In this case, I was behind a barricade in the dead center of the field (Biohazard in this case). I was in a great position, shooting at people left and right, keeping their heads down and preventing them from moving up.
While I called a ref to check on a gun hit I made, I heard fire behind me, thought it was a teammate. In fact it was someone from the other team shooting from the other side of my cover, not even realizing I was there.
I turned, tapped my marker against the side of his marker (didn't shoot), said, "got you dude, you're out." Well apparently he panicked, didn't know I was there, literally put his barrel into mine, shot in there, fell back, then stuck his gun around the corner and blindfired on me from a foot away
, hitting me in the leg (thank you Planet Eclipse pants!). Called out not to blind fire, fortunately he stopped, probably scared him with my tone. I maintained my cool, didn't want to start a fight with a newbie, but did inform the ref about it who promptly called him out.