This sentence can be interpreted as: a dry land, the drought has lasted for a long time, and finally ushered in a heavy rain that had not come for a long time. It was really timely. A bundle of dried firewood touched a large fire, and the dry firewood quickly ignited.
This is the original meaning of this sentence. The extended meaning is timely rescue and inevitable results, indicating that things are inevitable.
Extended information:
The four great joys of life
The four great joys of life: the dew that comes after a long drought, meeting an old friend in a foreign land, the night of flowers and candles in the wedding room, and the time when the gold medal is named. From "Poems of Prodigy·Four Joys", the author Wang Zhu, courtesy name Dewen, was born in the late Northern Song Dynasty. Wang Zhu composed poems when he was nine years old and was known as a "child prodigy".
Wang Zhu has great poetic talent. He has written many five-character quatrain poems, which are short poems that are easy for children to memorize. There is a compilation and interpretation of more than thirty five-character quatrains written by Wang Zhu, the school master at that time, titled "Poems of Wang Prodigy".
The lyrics of "Poems of Prodigy" are easy to understand and very suitable for children to memorize. It is known as "ancient and modern wonder book" along with "Three Character Classic",
Baidu Encyclopedia-Four Sorrows and Four Joys