Holograms are a type of buffer swirls. They're the kind that (generally) only show up in natural sunlight and are only visible from certain viewing angles. They (generally) have a "rainbow" type appearance. They're (generally) rather shallow and not that hard to remove. Take out those "generally"s and you can have some real problems on your hands, but fortunately that doesn't happen all that often.
There are radically awful holograms caused by the unskilled (often with harsh compounds and/or wool pads) that are similar but worse. Sometimes these can only be removed by better work with a rotary. Some people here have encountered these on hard clear (Audis) and been unable to correct them with the PC.
Note that the answer isn't always to just use milder pads/products and/or to "do the job right". Some folks here have had trouble with holograms even when a) they know what they're doing, b) were using mild products correctly. Every now and then it's just a
situation where a more gentle approach (PC/Cyclo/etc.) is the sensible solution.