King Tropism is a simplified form of king safety evaluation.
It takes into account the distance between the King and the attacking pieces, possibly weighted against piece value.
For example, one may double the distance value for a queen, and halve it for bishops and rooks.
This kind of evaluation acts in a probabilistic way - it is by no means certain that being close to the king helps in attacking it.
For example, if white castles short, black rook on h8 gets a higher tropism value regardless whether it stands on an open file.
King Tropism