To light or ignite (something); to set fire to (using a match, lighter, etc.).
キャンプで薪に火をつけるのに少し苦労した。
We had a bit of trouble lighting the firewood at the camp.
古いコンロにマッチで火をつけた。
I lit the old stove with a match.
乾電池と金属片で簡易的に火をつける方法があるが危険だ。
There is a risky improvised method to create fire with a battery and metal, but it's dangerous.
Figuratively, to spark or inflame a situation (to trigger an incident or incite strong feelings).
彼の一言が騒動に火をつけた。
His remark sparked the commotion.
SNSでの投稿が論争に火をつけ、話題になった。
A social media post inflamed the debate and became a hot topic.
その映画は人々の興味に火をつけた。
That movie sparked people's interest.