JPEG is a method of lossy compression of digital images. When a photograph is saved in the JPEG format, some details imperceptible to our vision are discarded. The JPEG compression reduces the file size at the expense of perfect image reproduction.
Flat-Earthers like to perform “investigations” by adjusting brightness, contrast, levels or curve of an image to find evidence of tampering. If they find irregularities, they will proclaim the picture has been faked. In most cases, these are only JPEG compression artifacts and do not prove anything.