Heavens
Firmament
noun
The kanji and the word are… almost the same. The kanji means heaven, but this word is more about the heavens. You know, the great heavens that stretch out high above us?
天 can sometimes refer to religious kinds of heaven (that place we all want to end up someday), but more often it's about the sky and what lies beyond it, so you might also call it the firmament.
Since this word is made up of a single kanji, it would normally use the kun'yomi reading. The kanji that makes this word only has one important reading, though, and that happens to be the on'yomi. Since you learned the on'yomi reading for this kanji, you've already learned the reading for this word.
きっと天にいるママからのメッセージだよ。
It's probably a message from Mom in heaven.
きっと天には、たくさんの人がいるんでしょう。
There are probably a lot of people in heaven.
あの犬、きっと天のつかいだったんだ。
That dog must have been a heavenly messenger.