What is plurality classification in decision trees?

They would have said the majority class if there were only two classes. Plurality is just the generalization of majority to more than 2 classes. It just means take the most frequent class in that leaf and return that as your prediction. For example, if you are classifying the colors of balls, and there are 3 blue balls, 2 red balls, and 2 white balls in a leaf, return blue as your prediction.

In decision trees when you have reached a leaf node but still do not have clear idea about the class to assign it to, then you have to return plurality classification, which means consider all the examples of the leafs parent and see the most common class occurred in the dataset.