This is the 666th most frequent Japanese word.
ずつ
"ずつ" means "each" or "apiece."
ずつ further expresses the 'one at a time' sense tied to each interval set by the sentence.
学期ごとに異なる科目を一つずつ学びます。
We study one subject at a time, differing each semester.
ずつ signifies gradualness here, reflecting the 'little by little' progression.
少しずつ努力すれば、成長できます。
If you make an effort little by little, you can grow.
ずつ is used here to indicate distribution, meaning 'each' or 'one by one'.