Hello Iris,
Welcome to the forums! :qiime2:
Great question!
Short answer: it's a Naive Bayes kmer classifier that's essentially like the RDP Wang classifier and the Mothur classifier.
Long answer: Feature-classifier explained in detail - #3 by Nicholas_Bokulich
Paper on classifiers in Qiime2 
Optimizing taxonomic classification of marker-gene amplicon sequences with QIIME 2’s q2-feature-classifier plugin | Microbiome | Full Text
Or maybe it's a good thing: Qiime 2 should offer better taxonomic resultsion than Qiime 1, so maybe this is a useful finding / correction to the original work 
Colin