This is the 13577th most frequent Russian word.
начнёт
"Начнёт" means "will start" or "will begin" in English.
'Начнёт' in this context refers to the beginning of a natural phenomenon, specifically rainfall.
Дождь начнёт лить через час.
The rain will start pouring in an hour.
The verb 'начнёт' here is used as the third person singular future tense form to denote that he (subject) will commence an activity.
Он начнёт работать завтра утром.
He will start working tomorrow morning.
Here, the verb 'начнёт' illustrates the act of beginning a scheduled event, performed by the teacher.