1Self-study — studying by oneself rather than being taught, typically outside of class time or at home.
図書館にこもって、苦手な文法を自習した。
I shut myself in the library and studied the grammar I was weak at on my own.
彼は独学で日本語を自習し、今では日常会話に困らない。
He taught himself Japanese through self-study, and now he has no trouble with everyday conversation.
自習で分からなかったところは、付箋を貼って後で先生に聞く。
For anything I don't understand while self-studying, I put a sticky note on it and ask the teacher later.
2Studying on one's own in the classroom, typically because the teacher is absent; a phrase commonly used in school settings.
先生が出張のため、5時間目は自習になった。
Since the teacher was away on a business trip, the fifth period became self-study time.
自習の時間はおしゃべりせず、静かに問題集を解いてください。
During self-study time, please don't chat and quietly work through your workbook.