"Folks" is basically an English version of the German word "Volk," which just means "people." So when someone says "Hello, folks!" they mean "Hello everybody."

The word has a bunch of other uses in American English. I may say "I went and saw my folks for the holidays." In this case, "Folks" means "parents."

Folk music is a genre of working-class traditional music in the United States. For instance, labor organizer and activist Woody Guthrie is a well-known folk musician, having written songs like "This Land is Your Land," and "All You Fascists Bound to Lose." Folk music is about working class life, things that everyday people deal with, and sometimes just fun or silly stories. For a long time, a subgenre of folk music written about science fiction or fantasy stories was prominent at sci-fi conventions or Renaissance Fairs, and is known as "filk," but this has sadly been on the decline as these events become more mainstream.

Someone who in typically informal in their speech and demeanor, sometime to the point of being seen as insincere, may be described as "folksy." A rustic restaurant that serves classic American style food my also be described as "folksy," as in "trying to appeal to everyday people and not appear sophisticated or fancy."

Modern Americans use "Folks" as a descriptor for a group of people because it is non-specific and gender neutral, as people try to be more inclusive. There's a meme where a tech employee is trapped in the workplace elevator, and tries to call for help on Slack, saying "Guys, the elevator is stuck, send help!" only to be chastised for not using a less gendered term such as "folks," so the next message is "FOLKS, the elevator is stuck, send help!"