Painful
Distressing, Strained
い adjective
This word ends in い, meaning you know it's probably an adjective. In fact, it ends in しい, which makes it even more likely it's an adjective. Since the kanji 苦 means suffering, this word is the adjective version of that: painful.
苦しい usually describes a painful or distressing feeling that's ongoing and hard to endure, rather than a physical sensation of pain. For example, you can use 苦しい when it's hard to breathe, when you feel emotional pain, or even when your finances are strained.
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:
Everything is really painful right now. Why? You have something heavy on top of you, crushing (くる) you.
食べすぎて苦しい。
I ate so much that I'm in pain.
一ヶ月前くらいから、このジーンズ、苦しいんだ。
These jeans have been uncomfortably tight since about a month ago.
あの時のことを思い出すと、今でも心が苦しい。
Remembering what happened back then still makes my heart ache.
村では、冬になると食べ物が不足して、村人はみんな苦しい生活を送っていました。
In the village, there was not enough food in winter, and all the villagers lived in hardship.