CRI is the Color Rendering Index. I don't claim to be a lighting expert, but the higher CRI, like 90, means the bulb is able to reproduce the colors it was meant to produce very accurately. I think all aquarium mh
bulbs are CRI 90. The lower Kelvin temp bulbs, such as those in the 3000k range usually have a lower CRI, around 80.
CCT is Correlated Color Temp. This the the actual kelvin temp of the bulb and what it will look like to the eye. Generally a 5000k bulb will look very yellow, a 10,000k white and 20,000K blue.
PPFD is Photosynthetic Photon Flux Density. This is the amount of photosynthetically available "light" in a defined area.