A 7D hologram is a method for capturing a high quality hologram using 7 parameters.
The universe exists in 3D space with time often considered a fourth dimension. The reason that a 7D hologram has so many dimensions is that the hologram is captured from a large number of positions that surround the scene or subject of the hologram.
Each position is described in 3D space. Each position captures a variety of viewing directions in 2D space. Two additional parameters are captured for each direction: image intensity and time. If you add these up you get 7 parameters, known as dimensions.