According to ScienceInsider, the question for Key Haven, Florida, reads:
Are you in favor of the Florida Keys Mosquito Control District conducting an effectiveness trial in Key Haven using genetically modified mosquitoes to suppress an invasive mosquito that carries mosquito-borne diseases?
The genetically modified mosquitos in question were developed by Oxitec and carry a gene that prevents them from reaching adulthood. By releasing male mosquitos with this gene in areas like south Florida, proponents hope to reduce the numbers of Aedes aegypti in the area and thus limit dengue, Zika, and chikungunya viral transmission.
Oxitec received permission to conduct a trial with their mosquitos from the US Food and Drug Administration, but because of local opposition, the Florida Keys Mosquito Control District hasn't approved the trial and instead has put it to a referendum.
ScienceInsider notes that though the referendum isn't binding, three of the five board members have said they will follow the voters' wishes.