1that; that thing — a demonstrative pronoun for something distant from both speaker and listener, or something understood without being named.
あれは何という建物ですか。
What is that building over there called?
あれを見て。山の上に雪が残っている。
Look at that. There's still snow on top of the mountain.
あれ以来、彼女から連絡がない。
I haven't heard from her since then.
あれは誰にも言わないでください。
Please don't tell anyone about that.
2that person; that guy — an informal way to refer to someone. It can sound dismissive or rude, depending on the context.
あれは私の弟です。
That's my younger brother over there.
あれに伝えておいて。
Pass the message on to that guy.
3then; that time — used to refer to a particular past time or event.
あれはまだ高校生だったころの話です。
That was when I was still in high school.
あれがあったから、今の自分がいる。
Because that happened, I am who I am today.
4that place over there — an informal use similar to あそこ.
あれが私の通っていた学校です。
That over there is the school I used to attend.
あれまで歩けば、海が見えてきます。
If you walk as far as that point, you'll start to see the sea.
5a colloquial euphemism for the genitals or down there, often written as アレ.
痛む場所を言いにくそうにして、「アレです」と答えた。
Finding it hard to name the painful area, he answered, Down there.
6a colloquial euphemism for a menstrual period, often written as アレ.
今日はアレで体調があまりよくない。
I'm not feeling very well today because it's that time of the month.