As often happens in the Scriptures, the term "latter rain" bears both a temporal & spiritual meaning. God desires to bless His people with the "former" rain moderately and then the "latter" rain allowing us to gather in the harvest intended for those who love our God.
"That I will give [you] the rain of your land in his due season, the first rain and the latter rain, that thou mayest gather in thy corn, and thy wine, and thine oil." (Deuteronomy 11:14)
"In the light of the king's countenance [is] life; and his favour [is] as a cloud of the latter rain." (Proverbs 16:15)