Behind
Back, Rear
noun
The hiragana attached to this word doesn't suggest it's a verb or an adjective, so we can assume it's a noun. While 後で (you may or may not have seen this word yet) means "after," this one means all the other meanings you learned with the kanji: behind, back, and rear. Think of the ろ as a hint. It's like saying you're on the "road" (ろ) and you're driving so fast you're leaving everything "behind."
うしろ
Since this word consists of a kanji with hiragana attached, you can bet that it will use the kun'yomi reading. You didn't learn that reading with this kanji, so here's a mnemonic to help you:
Continue to think you're driving on this road. You hit something, then look behind you, seeing you hit a cow. Know the word for cow in Japanese? It's うし, which is also the reading for this word.
あなたの後ろのドアをしめてください。
Please close the door behind you.
チャイルドシートは、かならず後ろのざせきにせっちしてください。
You must install the child carseat on the rear seat of the car.
振り返って後ろを見た後、さらにもう一度振り返ったら、僕のことが見えるよ。
If you turn around to look behind you and then do it again, you’ll see me.