Description
The apriori file gives a weight to the input classes that will be used during the classification. Each cell in the classification has a probability of being assigned to each class. The cell will be assigned to the class that it has the highest probability of being. In certain situations, you know that a cell is more likely to be part of one class over the other.
For example, if you are classifying a coastal area, it is more likely that a cell will be water than another land use. Since this apriori information defines a probability, to normalize the probabilities, they should add up to 1.
Solution
Make sure the sum of the apriori probabilities adds to 1. Check the Class Probability or Maximum Likelihood Classification documentation for further details on the formatting of the a-priori file.