To Insert, To Put In
transitive verb, ichidan verb
This word consists of kanji with hiragana attached. Because the hiragana ends with an う sound, you know this word is a verb. The kanji 入 means "enter," but this word does not mean "to enter" (which you already learned as 入る). Still, the meaning of 入れる is still related to the kanji's meaning: it means to insert or to put in. It's something you do to another object, like inserting a key into a lock, or your hand into a glove.
いれる
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.
毎日ボトルに水を入れる。
I put water in a bottle every day.
サンドイッチにアボカド入れた？
Did you put avocado in the sandwich?
ボランティアの会に入れて下さい。
Please let me join the volunteer meeting.