A threat is a virtual connection that a player (usually the player whose turn it is not) can create in a single move.
For example, in the following situation, Blue threatens to play at x and complete a virtual connection between Blue's edges (via a bridge and edge template II). If it is Red's turn, Red must try to prevent this connection. Red can do so, for example, by playing at x, y, or z.
The carrier of the threat is the set of cells that are required for the virtual connection to be valid. In the above example, the carrier of Blue's threat at x is the shaded area:
To neutralize the threat, the other player must play somewhere in the threat's carrier. (But not all moves in the carrier will necessarily be good).
If the carriers of two or more threats don't overlap, the other player cannot respond to all of the threats at the same time. But if the carriers do overlap, the other player might be able to neutralize all threats at the same time by playing in the intersection of the carriers. See mustplay region.