Units of mean decrease accuracy on a variable importance plot obtained from a Random Forests classifier?
Gini decrease is calculated based on the mean decrease in Gini i.e. $p_i(i-p_i)$ each time when the Tree is splitted on that Feature. Value is so high because the r package weight the impurities by the raw counts, not the proportions.
Accuracy decrease is calculated on OOB dataset by randomly shuffling the data for that particular feature in the OOB. Then subtracting the two accuracies i.e. before and after shuffling.
Unit -
Accuracy - Unitless - Ratio of correct classification to total records
Gini impurity - Unitless - Measurement of the cleanliness of split.Formal
These links have got good detail:
Stackoverflow
r-bloggers
Quora