RGB color model uses three colors RED GREEN BLUE and their intensities to create all other colors. Therefore, each pixel will have three channels each of which represents the corresponding color intensity. In other words, each pixel will contain its own individual color information (NOTE: this is true for an artificially created images, or images that have been obtained separately as red green and blue components, whereas most of images come from cameras that use color filter array, and demosaicing allows to reconstruct the missing colors - resulting in a pseudo-true rgb components).
I would guess that RGB 888 means that each color channel is 8 bit deep. Some censors have 12bit and other bit depths. So, I think RGB888 in not an HDMI standard but rather states the colorspace and the bitdepth. Some images in Windows have an alpha (transparence channel).
The other way to represent the color is in YUV colorspace - which is luma - the intensity, and the other two represent colors.
So, the two above and other standards (HSV, HSL) are used to represent the information about the pixel.
When we are talking about pixel stream, we can send the information about the pixels in many different ways. In some cases, we believe that we can send less information about the pixel and still reproduce a picture so the eye will not see the difference. Therefore, we use chroma subsampling - thus, instead of sending the full information about each pixel, we share that information with the neighboring pixels. I assume from your comments, that you understand how the subsampling takes place. If not, check out this answer.
Thus, the HDMI standard must specify how the color space standard (RGB, YUV...) and the subsampling (4:4:4, 4:2:2...). The YCrCb standard implies the YUV color spalce and CrCb or CbCr information order. The numbers after describe subsampling.
Now, this is how I would interpret the RGB4:4:4. The RGB stands for a 3 channel format (for each color). The 4:4:4 stands for no subsampling - that is each pixel contains its unique (original) information.