This is the 9711th most frequent Polish word.
zapalili
"Zapalili" means "they lit" or "they ignited."
Here, 'zapalili' is used as the past tense of 'zapalili' referring to the action of lighting candles.
Wieczorem zapalili świeczki na tarasie.
In the evening, they lit candles on the terrace.
In this case, 'zapalili' indicates the act of switching on the light.
Kiedy zapalili światło, zobaczyli niespodziankę.
When they turned on the light, they saw a surprise.
Here, 'zapalili' describes the lighting of a campfire for warmth.