In the game of Go (Wei Qi/Badouk), a situation called "ko" arises fairly frequently. Examine the diagram shown here:

......
..xo..
.x.xo.
..xo..
......

In Go, a stone or group of stones is captured if it is completely surrounded by the opponent's stones and/or edges of the board. If it was White's (o) turn in this diagram, she could play on the empty point in the middle of the Black (x) stones and capture Black's rightmost stone. Notice that now, the situation would be basically the same, only with White and Black reversed. What stops Black from immediately playing where his rightmost stone originally was and starting an endless capture/recapture loop is a rule called ko.

The ko rule is quite simple: it is forbidden to repeat a previous board position. So Black must first play elsewhere on the board, and allow White a chance to fill in the empty point, ending the ko.

If Black wants to recapture, and have his own chance to win the ko, his move elsewhere must threaten a follow-up move whose value is bigger than that of White filling in the empty point and winning the ko. If he does this, White will respond locally to Black's move, rather than filling to ko. Black's move is known as a "ko threat." After White responds, Black will capture the ko, and it will be White's turn to try to come up with an effective ko threat.

Therefore, a "ko threat" is a move which must be responded to, lest a local situation crumble. It is therefore related to, but not the same as, the notions of kikashi and sente. The main difference is that kikashi moves and sente moves are intended to gain something locally, while still retaining the initiative to play elsewhere. A ko threat need not gain anything, as long as the opponent responds. Indeed, some ko threats even lose points locally, and are still worth playing, provided the number of points that depend on the outcome of the ko is larger than the number lost by playing the ko threat. Below is a typical example of a ko threat.

........
.xx..oo.
..bxxxo.
.oxaoo..
.oxo....
..o.....
........

Black (x) has started a ko somewhere. White (o) plays at a, which doesn't gain much (maybe a point, depending on what else happens in the game), but Black must respond at b, or his two stones on the left will be captured. Note that the capture of two stones is only a difference 4 points (two prisoners + two points of territory). If the ko in question is worth more than that (for instance, if the life or death of a larger group depends on it), Black should ignore the threat, win the ko, and allow White to capture the two stones (since it will still be possible to save his three middle stones afterwards).

A common newbie mistake is to see moves which the opponent must respond to, and play them immediately, because "it doesn't cost me anything." In fact, it does cost something: It costs a ko threat. Zero-point moves should definitely not be played, even in sente, and should be held in reserve as ko threats. Even smallish sente moves should probably be held in reserve, until late in the game. Of course, loss of a ko threat is just one reason such moves shouldn't be played early on. Frequently, such moves are also aji keshi.