It just is an informal way to say "you". "Folks" is an informal work for "people" in this usage. It can in other usages refer to your family members, especially parents and more distant relations.
2015년 11월 29일
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It is an American informal term for a group or family members. Mostly only Americans use it.
You don't have to use it anywhere unless you want to impress or be friendly towards Americans.