Finishing reading something — completing the act of reading; can also refer to marking an item as read (e.g., in apps).
今朝、長編小説を読了した。
I finished reading the long novel this morning.
電子書籍を読了すると、進捗が自動で記録される。
When you finish an e-book, the progress is recorded automatically.
受信トレイのメールを全部読了にしてから退社した。
I marked all the emails in my inbox as read before leaving the office.