The wiki entry is not very helpful. It just says that it’s a small region of space that can deflect an orbiting body without providing any supporting science.
The first paragraph of that article isn’t that clear.
Essentially, there is a lot of uncertainty in the exact orbit of an asteroid coming near the Earth. They may know that it won’t hit the Earth at some time in the future (say five years), but be unable to rule it out at some later pass (say 25 years later). What they can do sometimes is calculate the range of orbits it could have to hit the Earth the second time, then work out where it would be be on the first pass for those orbits.
This range of locations is what’s called a keyhole. It’s smaller than the uncertanty of where the asteroid may pass by. If it passes by in the keyhole, its orbit is such that it will hit the Earth on the later pass. If it’s outside the keyhole, it will miss.
Note that the Earth is deflecting the asteroid whether it passes through the keyhole or not, it’s just that the keyhole is where will be if it will later hit the Earth.