Flute
Whistle
noun
The kanji and the word are exactly the same. That means they share meanings as well.
Since this word is made up of a single kanji, it should use the kun'yomi reading. When learning the kanji, you didn't learn that reading, so here's a mnemonic to help you with this word: Pretend you're trying to whistle, but you can't. All that comes out is air. You make a "Fuuuuuuueh" sound over and over, because you can't get the whole whistling thing down. Go ahead and pretend to do that (or do it for real).
その笛には、ツタが絡まっていました。
The flute was entangled in the vines.
このはと笛、お母さんがおまつりで買ってくれたの。
My mom bought this dove flute for me at a festival.
あの笛を持っている男の人がコウイチさんだよ。
That guy holding the whistle is Mr. Koichi.