The simple answer – MARKETING!
It has no realistic meaning. It is simply meant to get the attention of the soon-to-be-perplexed. All marketing, little to no substance.
Each dimension is a way of seeing or sensing something.
1D (one dimensional) is mostly a theoretical idea or the domain of the very small world of quantum mechanics. Everything we can see has 2 dimensions. Even a very very thin long line has at least some width.
2D a flat representation of a scene or object. its size can be described as height and width. Like a square, picture or image on a standard TV.
3D a recreation of an object or scene in three dimensions. So height, width and depth like a cube or sphere. We have two eyes so we have great depth perception and experience the world in 3D
4D is basically 3D plus movement over time. So Time is the fourth dimension
5D and above are not viewable or detectable by us in our Universe. These extra dimensions are the playground of fiction authors and theoretical physicists.
String theory predicts our block of 4 dimensions are only a small group of a much larger group of dimensions. This does not mean that there are perfectly formed replicas of the universe we can detect with alternate versions of us and different histories played out. If there are multiple dimensions outside of our own and we could never interact with them in any way then they might as well be not there as far as we are concerned on a daily basis. Of course we are a curious lot and we can’t stand being told no, so lots of theoretical scientists, authors and BS artisans are actively exploring the possibilities of the extra D’s. Each dimension is a way of seeing or sensing something.
4D , 5D, 6D are mostly marketing terms for tech that cannot sell on its own merits.
So the next time you go looking around for 7D holograms, please read this first.