Refers to writing something in, such as filling in a form, making an entry in a ledger, or adding notes in the margin. Often used in compound nouns like 書き入れ時 (peak business period).
I make entries of figures in the ledger.
The year-end is the peak business period for retail stores.
peak business season; busiest and most profitable period
Compound of 書き (kaki, continuative form of 書く 'to write') + 入れ (ire, continuative form of 入れる 'to put in'). Literally 'writing in'.