Penmanship or calligraphy practice. Often refers to school lessons or extracurricular practice of writing characters neatly.
小学校で習字の時間がある。
There is calligraphy/penmanship class in elementary school.
習字の宿題で半紙に名前を書く。
For penmanship homework, write your name on a sheet of paper.
習字セットを買って練習を始めた。
I bought a calligraphy set and started practicing.