This is the 423rd most frequent Japanese word.
ほとんど
Almost; nearly; mostly.
Here, 'ほとんど' is used as a noun modifying particle-phrase to express 'most of'.
ほとんどの学生が試験に合格しました。
Most of the students passed the exam.
Here, 'ほとんど' modifies '毎日', meaning 'almost' in terms of frequency.
この部屋にはほとんど毎日います。
I am in this room almost every day.
In this sentence, 'ほとんど' quantifies the noun '雨', expressing 'almost none'.