That luggage looks heavy.
This car is so heavy that my arm is about to fall off.
"I’m sorry I stepped on your foot." "No problem. It was a little heavy, but no harm done."
This is a single kanji with an い on the end, meaning you know it's probably an adjective. What's the adjective form of heavy? It's also heavy.
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:
You're lifting something that's really heavy. Oof! You lift it above your head. You're so awesome. You scream: "OH! MORE!" (おも) they put more. You yell it again: "OH, MORE!" and again: "OH, MORE!" Go ahead and act this out wherever you are, pretending like you're holding a couch or two or three over your head, saying "OH MORE!"