This technique has an effect of reducing the importance of words such as 'the', 'a', 'to' which can be predominant in a text corpus but are not as meaningful as other less frequent words such as 'city', 'capital', 'animal', etc.
word w has probability P (w) = 1 − t/f (w) where f (w) is the frequency of word w in the corpus and t is the frequency threshold.