Insertion means putting something into something else, or the thing that gets put in. When a dentist fills a cavity, that's an insertion of filling material into your tooth. When you slide a key into a lock, that's an insertion too.

You'll see insertion used in many contexts. In writing, an insertion might be a word or sentence added into a paragraph to make the meaning clearer. In sewing, lace insertion is decorative lace sewn between two pieces of fabric. In medicine, doctors talk about the insertion of a breathing tube or an IV needle. In biology, the insertion of a muscle is where it attaches to the bone it moves.

The opposite of insertion is extraction (taking something out) or removal. Think of it this way: when you insert a USB drive into your computer, you're doing an insertion. When you pull it back out, that's extraction.

The word can feel technical or medical, but it simply describes something we do constantly: putting things where they belong, whether that's inserting coins into a vending machine or inserting a bookmark between the pages of a book.