If you shoot and save your images in the JPEG format then you have already thrown away some of the original information that the camera captured when the shot was taken. This cannot be retrieved.
In most circumstances, this will not matter and the vast majority of high quality JPEG to JPG images produce highly acceptable and pleasing results.
Your camera itself may save the image as a JPEG in which case, its internal processing has already discarded some of the original information. If you then subsequently manipulate the file on your computer and then re-save, even more information is lost. As the file loses information, so the image becomes progressively degraded.