to insert; to put in
to send in (to a prisoner; e.g. clothes, books)
to deliver (food, refreshments, etc. to workers); to supply
鍵を鍵穴に差し入れる。
Insert the key into the keyhole
書類を封筒に差し入れた。
I inserted the document into the envelope
彼女に差し入れを贈ることにした。
I decided to offer her a gift.