In the context of what @tdubwilly and @zparkie2 have said, this is about how efficient the light is, 2 micromoles per joule.
Until recently HPS and CMH had about the best micromoles per joule.
It is kind of like miles per gallon, how much light can you get per the amount of power/electricity you are using.
You can get enough intensity with less efficient lights, you just will need to add more of those lights than you would of the more efficient lights, well kind of…
Or, you might need to buy a light that uses more watts to get the same results as the more efficient light.
It used to be, with HPS, CMH, or regular metal halide lights, you could do a rough calculation of how much light you’d need by using a formula like “50 watts per square foot”.
More micromoles per joule would mean you’d need less watts per square foot.
But that isn’t the entire story. LEDs and Florescent lights like the T5s a lot of people use, don’t create as much heat at the bulb as the High Intensity Discharge (HID) lights like HPS/MH/CMH.
The closer the light gets to the plant, the more micromoles of light energy or intensity the plant receives.
And so again, you can get away with less watts per square foot if you can get your light closer to the plant without hurting it with too much heat.
So a another rough formula would work out kind of like about 40-50 watts per square foot for T5s, and depending on the LED tech as low as maybe 20-25 watts per square foot or maybe as high as 35-45 watts per square foot for older tech LEDs.