〜ずつ
each, at a time
Number + counter + ずつ / 少しずつ
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ずつ after a number means "each" or "at a time": 一人二つずつ取ってください, "please take two each"; 一つずつ説明します, "I'll explain them one at a time".
It shares out an amount evenly (みんなに千円ずつ, "1,000 yen each to everyone") or spreads it over time (一日三十分ずつ, "thirty minutes a day").
少しずつ, "little by little, a bit at a time", is one of the most useful phrases in the language, and exactly how Japanese is learned: 毎日少しずつ.
ずつ never goes on its own; there's always a number or an amount before it.
Examples
一人二つずつ取ってください。
Please take two each.
毎日少しずつ日本語を勉強しています。
I'm learning Japanese a little at a time every day.
一つずつ説明します。
I'll explain them one at a time.
みんなに千円ずつあげました。
I gave everyone 1,000 yen each.
一日に三十分ずつ歩いています。
I walk thirty minutes each day.