This is the 1049th most frequent Russian word.
начнут
"Начнут" means "they will begin" or "they will start."
The word 'начнут' is the future tense third-person plural form of the verb 'начать', meaning 'to start'. Here, it indicates the action of beginning (construction) that will happen tomorrow.
Завтра они начнут строить новый дом.
Tomorrow, they will start building a new house.
The word 'начнут' in this sentence conveys the idea of beginning (to help) influenced by a condition (our starting the project). It demonstrates dependency in action timing expressed through future tense.
Как только мы начнем проект, они начнут помогать.
As soon as we start the project, they will start helping.
Here, 'начнут' expresses the future action of starting (playing) by the children following the completion of another event (the lesson).