A dead cell is a cell of which colour does not depend the outcome of the game.
Typical dead cells are useless triangles
In the following patterns, the dead cell is labelled with a star.
Because the colour does not affect the result of the game, dead cells can be completely ignored from analysis. In computer Hex, it can save a huge amount of time to check which cells need to be evaluated and which are irrelevant.
The concept of dead cells can also be usefulf in human play, since it can help telling in which side of a bridge it is better to intrude. In the following example, if we assume that Red wants to keep its pawns connected then it is far better to play in field c3 for Blue. It helps gainning ground to play in c3, whereas if Blue had played b3 then c3 from Red would leave it dead.