This is the 939th most frequent Japanese word.
とじ
"とじ" means "binding" or "closing" (e.g., of pages or documents).
In this sentence, とじ refers to the act of closing something, specifically a book.
本をとじました。
I closed the book.
Here, とじ is used in the context of securing something together, like sealing a letter in an envelope.
手紙を封筒にとじてください。
Please enclose the letter in the envelope.
In this instance, とじ is applied to organizing or attaching documents together in a file.