This is the 44th most frequent French word.
tous
"Tous" means "all" or "everyone" in English.
'Tous' is used to mean 'all' or 'every one' and refers to a group (students in this case). It agrees in number and gender with the noun it modifies, showing inclusiveness.
Tous les étudiants sont présents aujourd'hui.
All the students are present today.
'Tous' here acts as a pronoun meaning 'everyone' or 'all of them', used for emphasis and positioned after the verb.
Je les ai vus tous hier.
I saw them all yesterday.
'Tous' modifies the subject of the sentence ('Ils'), indicating that every individual in the group performed the action (smiling).