To catch fire; to be ignited (literally), e.g., a match or candle begins to burn.
マッチに火がついたら、素早くろうそくに移してください。
Once the match catches fire, quickly transfer it to the candle.
古いストーブから火がついて、家の一部が焦げてしまった。
The old stove caught fire and part of the house got scorched.
キャンプで焚き木に火がつくまで時間がかかった。
At the camp it took a while for the tinder to catch fire.
To flare up or ignite figuratively—an emotion, dispute, craze, etc., suddenly intensifies or takes off.
彼の一言で議論に火がつき、会場は大騒ぎになった。
His single remark set the debate alight and the venue erupted into chaos.
SNSで紹介されてから、その商品に火がつき一夜で売り切れた。
After being featured on social media the product took off and sold out overnight.
練習中の失敗がきっかけで彼の闘志に火がついた。
A failure during practice sparked his fighting spirit.