The kick is the small projection on the underside of a slip-joint blade tang that stops the blade traveling too far when it closes, keeping the edge from contacting the backspring inside the handle.
When a slip joint closes, something has to stop the blade before the edge hits the inside of the frame. That is the kick. It contacts the backspring and holds the edge clear.
It is why a well-made traditional knife can be closed thousands of times without the edge ever touching metal.
Every time you sharpen a traditional knife, the edge moves up slightly. Eventually, on a heavily sharpened old knife, the blade can sit deeper when closed and the edge begins to contact the spring. That is called the blade going past the kick, and it is a normal end-of-life signal on a much-used knife.
A cutler can sometimes correct it. On a modern folder the equivalent job is done by the stop pin.