What will you put in the square?
Please insert twelve pieces of origami paper here.
I put a hamburger into the coffin.
This is similar to the vocab word 入る (to enter), except in this case, instead of entering you're inserting something. So, it's like you're entering something into something else. You can remember that this one is to insert because it's longer than 入る (はいる/to enter). Why? Because it's more difficult to insert something than it is to enter somewhere, which is why you have more help (the extra kana stuck on the end).
When a vocab is a single kanji plus some okurigana (hiragana stuck on the end of kanji) it's almost always going to be the kun'yomi reading. This is one of two kun'yomi readings you'll be learning for the kanji 入. As long as you know that this vocab means to insert, you can use that to remember the reading as well. Imagine inserting your hand into a hole and an eagle (い) flies out, screaming: "い!"
This is something that happens every time you insert your hand into something: an eagle comes out. You don't know why.