Gentlefolk are people of the higher social classes — the well-born, the comfortably rich, the ones older books call ladies and gentlemen. The word is almost always plural. You will find it in stories set in the past, or in a writer putting on an old-fashioned voice. Nobody in a shop or on a bus says gentlefolk now. Though actually, folk in this word does not mean 'ordinary people', which is how students often meet folk. Here it just means 'people', and gentle once meant 'of good birth', not 'soft' or 'kind'. So gentlefolk are not 'kind people'; they are 'high-class people', at least in the speaker's eyes.

上流人士（舊時用語）