adverb
one by one; one at a time; in turn
Used when people do something individually, each in their own turn, or when distributing items to each person.
生徒たちは一人ずつ教室に入った。
The students entered the classroom one by one.
プレゼントを一人ずつに配った。
I handed out the presents to each person one at a time.
一人ひとり emphasizes each individual as a separate entity, often used for stressing individuality or addressing everyone. 一人ずつ focuses on the sequential or distributive aspect, doing something one person at a time.
Compound of 一人 (ひとり, 'one person') + ずつ (adverbial particle meaning 'each' or 'at a time').