Generally speaking jpeg to mac conversion represents any way how you might open or work with JPEG graphics on Mac computers and this can be done with many graphics tools for Mac, including the Preview app. But many users are looking for a particular thing - how to convert jpeg ton mac, which is of course also doable with the same tools.
A very specific case of jpeg to mac conversion represents the process of making new graphics files in ancient MacPaint raster format (.mac), which comes from one of the first graphics editors for Mac. Still doable in some graphics editors, but the support for that particular type of mac files is low.
